For Mac versions starting with Lion and some settings of Gatekeeper, you may receive the following error message when you try to run SPICE:
"spice" is damaged and can't be opened. You should move it to the Trash.
Click "Cancel". You will need to temporarily disable Gatekeeper, launch SPICE, and then re-enable Gatekeeper again (if you wish). You only need to do this once. Afterwards, spice will launch in the usual manner.
To temporarily disable Gatekeeper, open System Preferences. Under "Personal", click on "Security & Privacy". If necessary, click the lock and enter your password. Under "Allow applications downloaded from:", select "Anywhere". You may leave this window open.
Go back to the finder and double-click on the spice application to launch it. You will receive the following message:
"spice" is an application downloaded from the Internet. Are you sure you want to open it?
Now you can go back to the "Security & Privacy" window and set the "Allow applications downloaded from:" to whatever value you are comfortable with.
Whenever you wish to use SPICE in the future, you can just double click without altering Gatekeeper.