If your Mathcad 15 license doesn't work you will see a dialog box like this:
Click on 'Yes' to start the License Setup. You can also start the the Wizard manually in a Run box:
Or by opening the mclicense folder where Mathcad 15 is installed and starting mclicense.exe:
On the first page you choose the second option saying 'I want to configure Mathcad to use an existing license file":
You should the see:
Here you can click on the button with the three dots and browse for your license file. Maybe your doesn't have a .dat extension. Then just select to look for 'All Files (*.*)':
After finding your should see something like this:
Click Finish and you're done. Then start Mathcad 15 to see that it worked.