You can also trying running it straight from the EXE.
It should be located in,
C:\Program Files (x86)\Steam\steamapps\common\sins of a solar empire rebellion
However you may have it labeled as another Drive or moved all of your games to an external / SSD.
Also you mentioned not being tech savvy no problem, it should say "Sins of a Solar Empire Rebellion" , it's type is application and it's size is around 5 MB if you are looking at the columns next to it.