This is for if you are having issues adding an existing datastore to a new esx server. 
I know this works for ESX 3.0.1.  Go to Configuration -> Advanced Settings.
Then click LVM.  Then go to the field labelled LVM.DisallowsnapshotLUN and set it to 0.  Then click ok.  Now rescan your storage and it will show up.  Also if that din't work make sure you check your ACL is correctly configured.