Why is a ketone not capable of hydrogen bonding among themselves?
Short answer: the ketone oxygen can participate in hydrogen bonding

The compound you mention, methoxymethane, is an ether not a ketone.  



Ethers don't hydrogen bond very well because the oxygen isn't very polarized.  

On the other hand, the carbonyl in a ketone is polarized.  We can draw resonance structures that show this polarization.



(image source)

The negative polarization on the carbonyl oxygen allows it to play the role of hydrogen bond acceptor as illustrated in the following drawing.



(image source)

Edit: If you are asking if the hydrogens on the carbon adjacent to the carbonyl carbon (or any other carbon in the backbone of a simple ketone) can participate as the donor in a hydrogen bond, then the answer to this question is "no". These $\ce{C-H}$ bonds are typically less  acidic and less polar than the $\ce{O-H}$ bond in water.

Therefore, while the oxygen in pentanone can participate in hydrogen bonding with donor hydrogens in other molecules, the hydrogens in pentanone cannot participate in hydrogen bonding.  Pentanone cannot hydrogen bond with itself.