You N E E D to have Blue Shift/Opposing Force installed for this to work.

This cannot be done in-game, it can only be done by manually going into the files.
Find the side bar right below STORE and LIBRARY
Right click on Sven Co-op and a bar will show up, it looks like this:
Next hover your cursor (or arrow if ur not a computer nerd) over Manage and click on Browse local files.

Once you click Browse local files,Windows File Explorer will open up to this:

Click on the folder titled "svencoop"
Once you open it, if you scroll down you will find 2 files:

[*Install_bshift_Support.cfg] [*Install_OpFor_Support.cfg]

If you were thinking you would click on thing, you were right! but wrong at the same time.
These files are b̷r̶o̷k̷e̸n̴, so we need to fix them!
If you scroll back down you will find "FindHLDirectory.cfg"
Right-click on that file and press Edit as a button that comes up, once you do this will pop up:
You will find a part titled "VALUE_NAME=ModInstallPath"
Change this to "VALUE_NAME=SourceModInstallPath"
Save and exit.

Browse local files of Half-Life and since you MUST have Bshift and OpFor you will find 4 files:
bshift
bshift_hd
gearbox
gearbox_hd
Copy all of these files
Navigate to C:\Program Files (x86)\Steam\steamapps\sourcemods
Paste all of these files.
Now those other install files are fixed and ready to go!
Navigate back to svencoop folder and double click both/a file(s).
You have successfully installed Bshift/OpFor/Bshift and OpFor! Now go play those extensions with ur friends. DON'T FORGET TO RUN FINDHLDIRECTORY.CFG