for example:
C:\Users\Dirk Rockface\AppData\Local\Plutonium\storage\t6

you can get there by typing %localappdata% in the windows search bar in the task bar/start menu... it'll look like this:

then you can click on that folder in the top and navigate the rest of the way...

the "mods" folder goes in the storage\t6 folder (if it's not there, then you can just create one)