When you first log into Challengermode, there are two highlighted menu items in the left sidebar navigation.
Click on "Connect Steam" to add a game account for Counter-Strike.
Add a Steam account
By clicking on the Steam button you will be redirected to the Steam account verification page (outside Challengermode).
Simply verify your account there (log in) and you will be redirected back to Challengermode. You'll know when you're done if you can see your Steam account like so: