The paths found in Momentum's gameinfo.txt for Counter-Strike: Source need to match where you have the game installed. By default it assumes the C:/ drive, under Program Files, etc. But this might not be the case.
So what you need to do is find where you have Counter-Strike: Source installed, and then open momentum/gameinfo.txt
, and find these lines:
[upl-image-preview url=//i.imgur.com/mYU43Mw.png]
You'll want to edit this path in the quotes to where you have Counter-Strike Source installed. So, for example, if my CS:S is found in:
E:\Games\common\Counter-Strike Source\cstrike
I would change the lines to be:
game "E:\Games\common\Counter-Strike Source\cstrike\cstrike_pak.vpk"
game "E:\Games\common\Counter-Strike Source\cstrike\download"
game "E:\Games\common\Counter-Strike Source\cstrike"
Note that you can also do the same for Team Fortress 2, if you plan on playing any jump maps.
Hope this helps!