Most mapping projects hosted on Map Channels can be added to your site simply by copying and pasting the embed code. These maps and do not require an API key.
You need to get an API key if you are locally hosting a Version 5 Map Channel or Version 4 Map Channel or a Venue Map.
Visit the Google API Console and log in using your Google account.
When you select the list of sites where you key can be used please select mapchannels.com. Then click on the 'My Profile' button in the top right and set your API key in your profile.
Edit the HTML of the web page and find the line containing maps.google.com/maps/api.
Add your key to the map api script line line using the ?key parameter.