You have the download. You installed it. Now you need a license file (license.dat).
You cannot legally activate Mathcad 15 for free. The "hot download" cracks you see on Reddit or obscure forums are riddled with malware. Instead, consider these alternatives: ptc mathcad 15 download hot
Call PTC Support (but note they charge a "legacy recovery fee" of approximately $199). You must prove purchase with an old email or receipt. You have the download