Ubuntu – Guest Additions not working

I am having really annoying problem and have spent hours trying to fix. I have searched ad nauseum and found others with same problem but none of the solutions work. I go to Devices to Install Guest Additions and find that instead it says Insert Guest Additions CD. When I try that it says it cannot insert and asks if I want to force mount. I try that and I get error that it cannot force unmount because it is locked (I get an error that is similar to this). What the heck is going on? Appreciate any advice. Thank you!