Instructions
Open Project 64 by selecting it from your Start menu.
Click the "File" tab at the top left corner of Project 64 and select "Choose ROM Directory." Select the directory where your "Mario Tennis" ROM is saved and click "OK."
Click the "File" tab again and select "Open ROM." A list of available games will be displayed. Double-click "Mario Tennis" to open it. Your "Mario Tennis" ROM will now load.