Is it a legitimate copy? if not just go to alterIWnet's website, they have a copy so that you can play online with other alteriwnet users and you wont need MW2 liberation or steam but still can host mods.
Liberation was made only for steam if I'm not mistaken.
Has it worked for you before? Have you successfully launched MW2 through Liberation before?
Try redownloading Liberation, also what version is your liberation.