Yes that is the right way to get the estimated execution plan.
Can you display execution plans for other stored procs?
Also try getting the entire text of the stored proc into an SSMS window,
delete the create proc line
add 'DECLARE' for each parameter, and delete the 'AS'
then highlight the code and display estimated execution plan. If you still get the failure try selecting the declares plus the first statement etc, breaking it down to see which statement is at fault.
Also - the obvious - is the proc calling any CLR procs or CLR UDFs?
And - does it work if you enable the clr?