This gets me every time I try to do it: it's much easier for me to create all but the most simple of zpool setups via the command-line zpool command than to use the FreeNAS GUI. But, if you do that, the GUI has no idea that the pool exists. There's a tantalizing "Import Volume" option, but if you don't remember the trick, it never finds anything.
The bit I always forget is to run zpool export on the pool in question. Then you can use the "Import Volume" tool — it finds the volume, and updates whatever bits FreeNAS uses internally to keep track of things.