Use the following trick to unlock all the levels in the campaign mode. Note: This procedure involves editing a game file; create a backup copy of the file before proceeding. Use a text editor to edit the "*.usr" file in the folder with the profile and save game directories. The *.usr file contains information about the missions and other game options. You will find some lines like this:

*VARIANT 1 0
*VARIANT 2 0
*VARIANT 3 0

The first number is the difficulty of the stage, while the second one describes its completion. To unlock all the levels, edit the lines to resemble:

*VARIANT 1 1
*VARIANT 2 1
*VARIANT 3 1