Bought a DLC but when try to install it says cannot find base game?

[lt1956_slith]

I have the base game and several other DLCs that I downloaded years ago. I though about buying Allies resurgence and when I tried to install and activate the serial given, I get an error message saying it cannot locate the base game, even though it is on my Win7 PC and I have been playing it. I wish the installers were made where you can pick the location as mine is in C:\Program Files (x86)\Slitherine.

I wasted my money on a purchase I cannot play because of the installer. I believe my version of the game is the Slitherine version, not GOG or NOT steam as I cannot use steam on Win7 setup