Mathcad

I just installed Mathcad 15 and I also bought Roarks worksheets from PTC installed in my computer. But all XMCD files in folder shown as Adobe , i can't open them when I click them, and I only open them in Mathcad 15 environment. I want to open them directly when I click a XMCD file, how can I do?

Thanks,

Ling