Instructions
Download the Dark Knight bat suit mod file from Eidosgames.com and save it to your computer. The file name is "CaptainJacks dk suit (enhanced).tpf".
Download Texmod, an application that allows you to run and implement mods, from Download.chip.eu. Launch the program by double-clicking on the downloaded file.
Click the button marked with a folder icon under "Target Application."
Go to "C:\Games\Eidos\Batman Arkham Asylum\Binaries\ShippingPC-BmGame.exe" on your computer. Replace "C:" with the letter of the drive where you installed the game, if you installed it on a drive other than your primary hard drive.
Click "Package Mode" in Texmod.
Click "Open File" and select the Dark Knight bat suit file you downloaded in Step 1. Click "Run." The Dark Knight bat suit will be added to the game for you to use while playing.