There is the Settings application, scroll down to Product Information, look for the Version number to find your PR.
Anyway, as you are not able to update via the N900, i would suggest to contact Nokia Customer Care. Did you buy it in a local store or some online service, they should tell you about possible restrictions to download data? Do you have a PC for trying to update over the Nokia-Software, but i never used that application at all.
Depending on your computer knowledge you might consider reflashing http://wiki.maemo.org/Updating_the_tablet_firmware the whole thing.