There are two main reason why you might receive this error message..... the first and most uncommon is you are running a lot of addons that use the FSUIPC hotkey facility and have exceeded the maximum hotkeys allowed. Second, and more common, is you have another recurring problem and you are restarting FDC three or four times and the error continues to display a message until eventually you get the 'not enough hotkeys'
So, assuming the most common cause.....
1. Do you have the latest version of FSUIPC installed?
2. What version of FDC are you running?
3. Where have you installed FDC?
4. What version of Windows are you running?
5. Has FDC ever worked for you?
6. Do you see any other messages before the hotkey message appears?