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:
Please note: If your Privacy Settings on Steam are set to Private, you will not be able to link your Steam account. After making it Public it may take up to 15 minutes until you can link your Steam account to Challengermode.