The correct path is /sys/devices/platform/omap/tsc. If analog input were enabled, you would see files ain1, ain2, etc. in that directory
You need to load a kernel module in order to enable ADC on Ubuntu, see: https://groups.google.com/d/topic/beagleboard/DV0pROItT4w/discussion
You might also need a more recent kernel. I don’t know when ADC supported was added to Ubuntu, but I think it was after 3.2.18. You can update the kernel by downloading and running the “Stable” version of install-me.sh script at http://rcn-ee.net/deb/precise-armhf/LATEST-omap-psp if you have the “hard float” image of Ubuntu 12.04, or http://rcn-ee.net/deb/precise-armel/LATEST-omap-psp for the regular flavor of 12.04.