Currently you will need to run iBuilder on a Windows machine and then copy the created bundle over to PwnageTool (Mac) for use.
Download iBuilder from here to your desktop.
Extract the downloaded zip (iBundle1.02b.zip) to your desktop.
Double click iPhoneBundleBuilder.exe from the extracted folder (iBundle1.02b).
iBuilder will launch and display a list of sources and packages
To add a source enter its url into the entryfield then click the Add Source button.
Notice the new source is added to the Sources listbox.
Double click a package to add it to the User Packages listbox. Once you have added all the packages you desire, click the Build! button.
When your custom bundle has been built it will be saved to the Desktop in a folder called iBuilder.bundle. Click the OK button.
To use this bundle you must transfer it to your Mac. Hold down the CTRL key and click the PwnageTool icon. Click Show Package Contents from the popup menu.
Now navigate to Contents > Resources > InternalPackages and copy the iBuilder.bundle into it.
You can now follow this tutorial to not only add custom logos to your firmware but also include the iBuilder.bundle
THANKS***: A big thanks to Rotem925 for creating this application. You can support his work using the donate button in iBuilder!