PLEASE NOTE: Alfred is currently in beta and as such, these help pages are currently being developed.
In the few isolated cases, Alfred will be unable to find what you are looking for - This can be due to the OS X Metadata index being corrupt or disabled.
Open Preferences > Experimental and click "Rebuild Index". This will open Terminal and run mdutil to rebuild your metadata index. This may take over an hour, but will work in the background without affecting your ability to use your computer.
To set your hotkey, go to the Alfred preferences and view the General tab. Type your key combo (ctrl, alt or cmd + space/character) to set it.
If cmd + Space doesn't work, that hotkey may be assigned to another feature. This is particularly applicable if you use multiple languages on your keyboard.
Here's the procedure you will need to go through: