Depending on the version it was developed in, you can't just upload a PV+ program and edit it. It's not like a regular PanelView. You will need the original .APA file that the .MER was compiled from.
There is an exception if it was done using version 5.0, then you can decompile the .MER file using the application manager. This is not perfect either, we've had a few experiences where parameter files became fubar'd after the decompile.
It is possible to decomplie older versions using some AB directed "tricks", however parameter files will be lost and so will the alarms.
It's best to get the original .APA file if you can, saves a lot of hassle.