PTC software is not like other software 
You are not able to open files created with version mathcad 15 or below in Prime. You have to convert those files first to the Prime format. To be able to do so, you must have a full installation of the latest Mathcad 15 on your machine. You can download a 30day trial version you may use to do so.
If you have only very few files to convert you may also ask here in the forum for someone doing the conversion for you.
I guess you have the latest version 3.0 of Prime Express. If not, if you have Prime 2, you must state this in your request for conversion here, as people having installed Prime3 will not be able to convert to Prime2 format and a Prime 3 file cannot converted to version 2 format at all. You know, PTC software is not like ....