Hi Novly,
An Individual copy of AutoCAD Mechanical has a different product key than a Suite and its serial number is tied to that product key. Therefore, you would get an "invalid serial number" error if you tried to activate your ACM serial number with the Suite installer.
You can download the full version of AutoCAD Mechanical from Matt, our Virtual Agent. Click here and select the option Downloading and Installation -> Software Download Links -> I Need to Redownload my Product; then simply choose the program and version that you wish to download.
Best regards,
Camilo Lemos