Frantix Codes -

Cheat mode
Hold + at the level selection screen to display the code entry screen. Then, use the to enter one of the following codes. Press to accept and activate the code. 

Level select
Enter LVLANY1 as a code. 

Disable level select
Enter LVLANY0 as a code.

Invincibility
Enter INVINC1 as a code. 

Disable invincibility
Enter INVINC0 as a code.