Most likely what is happening is the Snapshot is online but it's not writable. In order to mount a snapshot of a VMFS filesystem, as you correctly pointed out, you'll need to Re-signature thus the snapshot needs to be writable as well. What you want to do is:
1) create a clone from the existing Snapshot
2) Host Rescan
3) Add Storage
4) Select the cloned volume
5) Add new Signature
The snapshot will mount as "Snap-xxxxxxxx-Original-Datastore-Label"
Assuming you're running ESXi 4.0 or above and the LUN is a clone of an existing VMFS datastore and not an RDM, you should have 3 options: a) Format b) Assign New Signature c) Keep existing Signature
Here's the process
Additionally, you may want to place a quick call into the Support center. It should be pretty straight forward to get to the bottom of this.