How to retrieve Dalton value from m/z value?
IUPAC is wrong

Martin's answer is useful for its pointer to the IUPAC Gold Book, which is one of the most authoritative sources for chemical nomenclature around.

However, on this particular question, the Gold Book is laughably wrong.


  The abbreviation $m/z$ is used to denote the dimensionless quantity formed by dividing the mass number of an ion by its charge number. 


This sentence is wrong.  The $m$ in $m/z$, as used by nearly every practicing chemist in the world (except IUPAC), is the ion mass, not the mass number.  The mass number is the sum of protons and neutrons in a nucleus.  Therefore it is always an integer.  Thus, according to IUPAC, ions with a charge of $\pm 2$ would always have $m/z$ values that are either integers (if $m$ is even) or half-integers.  Thus, according to their definition, saying the $m/z$ value of $\ce{C7H7^2+}$ is 45.52683903 would be wrong.  However that is exactly what most mass spectrometrists would say it is.

Here is some evidence: 


A lipid maps database entry for $\ce{C7H7}$ lists $m/z$ values for a hypothetical $\ce{C7H8+}$ ion that have four decimal places.  
I happen to have the latest print edition of the Journal of The American Society for Mass Spectrometry in front of me, and paging through the contents I see things such as:


  ...an intense signal for the $\ce{Cu^2+ - His_3}$ complex at $m/z$ 245.6, instead of the $\ce{[His_3 + 2H]+}$ at $m/z$ 215.1...
  
  ...MS spectra of standard solutions containing (a) lactose ($m/z$ 365.11, $\ce{[M + Na]+}$; $m/z$ 381.309, $\ce{[M + K]+}$), (b) cytidine ($m/z$ 266.218, $\ce{[M + Na]+}$, ...


These excerpts are not using the (integer) mass number in their calculation of $m/z$, they are using the ion mass, measured in Daltons.  (If they were, the only numbers after the decimal point would be .0 or .5).  That means that $m/z$ values are not really dimensionless, which makes the sentence wrong again.  In fact a unit called the Thomson has been proposed for $m/z$ values in honor of JJ Thomson. However this nomenclature has not really caught on widely.  Nonetheless, $m/z$ values are not dimensionless.


The Gold Book goes on...


  It has long been called the mass-to-charge ratio although $m$ is not the ionic mass nor is $z$ a multiple or the elementary (electronic) charge, $e$.


This sentence is wrong because $m$ is the ionic mass (in Daltons) as explained above.  It is additionally misleading because $z$ is always a multiple of the elementary electronic charge, with the understanding that charge must be measured in multiples of $e$, not in Coulombs.


  The abbreviation $m/e$ is, therefore, not recommended.


Finally something I agree with IUPAC on!


  Thus, for example, for the ion $\ce{C7H7^2+}$, $m/z$ equals 45.5.


Most folks would find that to be an OK approximation to the true $m/z$, which as I said above should be 45.52683903.    

For the purposes of your question, Martin was right when he said 


  For all intents and purposes the mass number may be seen as the mass of the ion in u or Da...


One thing to keep in mind is that this definition of "mass number" is at odds with IUPAC and nearly every other chemical text, which restricts "mass number" to be integers.

One additional complication

It's a bit unclear in your question if you want to determine the mass of an ion or the mass of the parent (usually neutral) molecule that gave rise to that ion.  If you want to determine the mass of an ion, then the other answer is right and you can just multiply the $m/z$ by $z$, the (absolute value of the) reduced electric charge.

If you want the mass of the parent neutral molecule, you will (a) need more information, and (b) have to do a bit more work. The extra information you need is the ionization mechanism.  In electron ionization mass spectrometry, ionization is usually by electron gain or loss.  So to get the neutral molecule mass, you will have to add back the missing electrons (or substract the gained electrons) from the calculated ion mass to get the molecule mass.  For example, if $\ce{C7H7^2+}$ arose by the loss of two electrons, the $m/z$ would be 45.52683903, the ion mass would be 91.05367806, and the molecule mass would be 91.05477522.  

Other types of mass spectrometry involve ionization by gaining or losing protons, not electrons, and so the difference between ion mass and neutral molecule mass is much more significant.