If your trial "ended" that means you haven't installed your license.
If you made your purchase within the past 14 days, go to your email and save the MJR file they attached to the emailed receipt, and then open it. This will launch MC18 and install the license. The MJR file is the "license installer" (not the application installer).
If you made your purchase more than 14 days ago, the MJR file they mailed you will be expired. In that case, use the "Restore from Internet" option given when you launch MC, and it will take you to a web page where you can request a new MJR file. When you do so, the MJR file is given to your right in the browser, and also separately, emailed to your registered email address. You can either open the MJR file right there (choose Open) in the browser dialog, or you can go to your email, get the MJR file attachment, and open it.
If MC18 isn't installed at all, go to the "sticky" thread at the top of this board, and download the installer EXE file and run through it first. This is unlikely because you're saying it says the trial is expired.
I have no idea what the "not compatible with Windows 7 64-bit" thing was. The only thing I can think of is that your browser was doing something weird with the MJR file. Just save it somewhere on your computer (on the Desktop or in your Downloads folder) and then double-click it there.