Create a new directory with "Profile0" name in your "c:\Program Files\Codemasters\Perimeter\Resource\Saves\" directory.

Create a new file with text editor, called as "data" (without extension!) in your "Profile0" directory.

Just copy and paste the following lines into your "data" file:

[General]
name=ABBYCHEAT
lastMissionNumber=28
difficulty=1

Save your file. Now you have access to all missions in campaign/battle mode.