I have a Samsung Galaxy Tab 2 7.0.
I have files on an external SD card but I cannot mount the tab as a usb drive from my computer - I cannot find a way to turn on USB Mass Storage.
How do I do this in JellyBean?
Starting in 4.0 (actually 3.0, but few devices used that), most Android devices use the MTP (Media Transfer Protocol) class rather than the Mass Storage class. This was done for various reasons, including the annoyances attendant to having to unmount the SD card from the Android device to access it from the PC.
Samsung has embraced this new way rather fervently and doesn't allow mass storage at all under normal circumstances (If you have root, you can force it on some devices. Not sure if it works for a Tab 2).
MTP mostly "just works" on Windows since XP and on Mac, though getting it working on Linux can require a bit of effort if you don't use GNOME. The Arch Linux wiki has a useful article on the ways and means of accessing MTP devices on Linux.