Try the following:
1. For 64-bit. Start a command prompt as administrator and run (cmd): C:\Program Files (x86)\Mathcad\Mathcad 15\Mathcad.exe /register
2. For 32-bit: C:\Program Files\Mathcad\Mathcad 15\Mathcad.exe /register
Try the following:
1. For 64-bit. Start a command prompt as administrator and run (cmd): C:\Program Files (x86)\Mathcad\Mathcad 15\Mathcad.exe /register
2. For 32-bit: C:\Program Files\Mathcad\Mathcad 15\Mathcad.exe /register