What are ISA +10, ISA -15 and ISA -30?
First of all:


  I have found out that an ISA atmosphere is:
   15 degC temperature 05 degC dewpoint 1013.25 pressure


Not entirely correct. The temperature and pressure are ISA values at mean sea level. These alter with altitude.
Dewpoint is not defined in ISA, as ISA does not contain dust, moisture and water vapor.
ISA-30 atmosphere means temperature deviation from ISA temperature. This means that on a given altitude, temperature is 30 degrees colder than in ISA. Respectively ISA+10 states 10 degrees warmer.  

Below is a table of ISA values.


For more physics for calculations, here is a very nice pdf on the matter.