The community will undergo maintenance on October 16th at 10:00 PM PDT and will be unavailable for up to one hour.
You have to have Mathcad 15 installed on the same machine as Prime (1 through 6), or you must be using Prime 7 (which doesn't require Mathcad 15).
Then you must have Prime licensed also (could be trial license), but once that's expired and you are thrown (back) to Express mode, no conversion is possible.
Success!
Luc