Level Unlockables
At the Password screen enter the following codes to unlock the corresponding level

Unlock Level 1.2:
Enter the code CBBCBBBB

Unlock Level 1.3:
Enter the code BDBFBBBC

Unlock Level 1.4:
Enter the code CDBFBBBD

Unlock Level 2.1:
Enter the code LBCKBBBF

Unlock Level 2.2:
Enter the code MBC3BVBC

Unlock Level 2.3:
Enter the code LDGYCVBC

Unlock Level 2.4:
Enter the code MDGYFVBB

Unlock Level 3.1:
Enter the code LBB?TVBB

Unlock Level 3.2:
Enter the code MBB7KVBL

Unlock Level 3.3:
Enter the code 4DB7KVBL

Unlock Level 3.4:
Enter the code SDB7KCBL

Unlock Level 4.1:
Enter the code 4BG7KCBL

Unlock Level 4.2:
Enter the code SBG7KCBL

Unlock Level 4.3:
Enter the code 4DG7KMBL

Unlock Level 4.4:
Enter the code SDG7KMB4

Unlock Level 5.1:
Enter the code 4BBY3MC4

Unlock Level 5.2:
Enter the code SBBY3MF4

Unlock Level 5.3:
Enter the code 4DBY3MK4

Unlock Level 5.4
Enter the code SDBY3MT4

Unlock Final Level: Scooby versus Masked Guy
Enter the code 5DBY3MT8


Unlock Alternate Ending:
Beat the game and when prompted for a code enter SD2