GT I9300 clockworkmod Cyanogen 10 nightly external SD not mounting
First to the recovery:

Samsung phones that use Odin have their recovery image as part of the kernel. You should flash the CyanogenMod zip file, and that should flash the CWM recovery. Alternatively, you could flash the Siyah kernel, which comes with a modified CWM touch recovery image.

Now to the EXT partition:

Make sure that the EXT partition is primary, and also ensure that your ROM has EXT support - you should ask on your ROM's forum post/website to check if it does. If it does, then it could be an issue with the mount script, if it doesn't well we know what the issue is now ;)