OK, so I just created a brand new (non-indial range) number and used that as the "Park" number.
I had to read the handset user guide in addition to the MCD help file before it clicked - (Correct me if I'm wrong)
To park the call you can:
key in "*51" then dial the "Park" number
OR
Configure a "Call Park" key on your handset, so you can press the "Call Park" handset key, then dial the "Park" number.
OR
When you configure the "Call Park" key, you also add the "Park" number as the destination number, so then all you have to do is press the key to park the call.