Did you bind a key? How it works is, you have to press a key on your keyboard to open it, but you have to tell it what key you want it to use.
Go to the options menu and click the controls tab. Scroll down the list of controls until you find the in game mod manager and double click it. Then press a key on your keyboard that doesn't already open something else. From then on, by pressing that key the in game mod manager will open.