Though it is slightly complicated, it is quite easy once you learn how to do it.
Lets say you have a tower with a base damage of: 5
Number of dice is lets say: 2 (There is a max of 100 here)
Sides per dice is lets say: 5 (There is a max of 100 here)
You will get: 7 - 15 damage
First, it does your base damage plus the number of dice ( For the 7 ) and then it does your number of dice x the sides per dice.(It gives 10 in this example ( 2 x 5 ) It adds that second number to your base dmg to give your second dmg value, 15 ( 5 + 10 )
I hope this helps you and i hope you understand what i wrote.