Just thought a lot of people might be wondering this with the recent sale on steam.
1. Start the game and copy the Zombie Island CD Key to your clipboard. 2. Find a New-U station anywhere in the game 3. Select "Jakobs Cove". It will start loading Jakobs Cove. 4. Press ALT+TAB to downsize the game. You should see a pop up on your desktop. If not, maximize the game again and you will see it. 5. If your mouse is glitching, use tab to find your way to the "Next" button, enter your CD-Key with ctrl+V for paste, then submit. 6. It will say it is verifying your CD-KEY and show a status bar. For me, the status bar would show no progress or activity for about 20 seconds. Then it would suddenly go from 0-100%. 7. You will then load into the new DLC area.
Repeat the same steps but for Moxxie's DLC key, and select the Underdome instead of Jakobs Cove.
"Uwee hee hee! Don't tease the octopus, kids!" -Ultros