Apple - How can I use my iPod touch as an external disk?

I don't think it is possible using iTunes.

You can try iPhone Explorer, though (which is free), or Phone Disk

You’re correct that other iPods had the ability to “Enable Disk Mode”, however:

Other iPod’s have Disk Mode where you can use the iPod as a hard drive. However, the iPod Touch doesn’t have this mode. There is currently no way to get it to show up in the Finder on a Mac or on the Windows Desktop. You are only able to use the iPod Touch within the iTunes application. (source)

Not all is lost, as there are some utilities that may do it.