Alright this is what I did to install the "custom-release" Angry Birds level pack.

These are the same instructions as the creator posted but are more Noob friendly.

The entire process once followed exactly won't harm your device but I still warn you to place no blame upon me or anyone if anything is to go wrong. I will however try to help where possible.

Well firstly, download the "custom-release.gz" folder to either your PC or to MyDocs on your N900.

When this is finished you'll need to extract the contents and to do this we use X Terminal. So if the folder is on your PC transfer it to MyDocs on your N900.

Open up X Terminal and get rooted (you must have rootsh installed to do this). To get rooted type "sudo gainroot".

Now you got to change the directory to where you placed the "custom-release.gz" folder. If you placed it into "MyDocs" then to change/go into the MyDocs directory type "cd /home/user/MyDocs" (spell exactly as shown)

Now you're working in MyDocs and you want up extract the folder "custom" from "custom-release.gz". To do this type "tar xzf custom-release.gz".

Now you want to be in the new "custom" folder which was extracted from "custom-release.gz" so type "cd /home/user/MyDocs/custom or just "cd custom". To verify if "custom" is in MyDocs use Cute Explorer or your File Manager to browse MyDocs.

Now we are almost finish. Type "sh setup-cust-AB" finally "sh run-cust".

Now the level pack you originally had has been replaced with the custom levels.
To go back to the original ones go back into X Terminal and get rooted again then change directory to custom (by typing "cd custom") and then typing "sh run-rovio". If you didn't close off X Terminal after getting the custom levels then just type "sh run-rovio" to revert/go back to the original Angry Birds levels.

It is suggested that before you perform an OS upgrade or buy and download the OFFICIAL LEVELS to go back into "custom" (cd custom) and run the command "sh run-rest".