Have you downloaded PAF from the LDS site yet??Once you have PAF installed on your computer, then you can open the email attachment from within PAF.
The first time you open PAF, once it's installed, the program won't know where your PAF database files are located, so will prompt you to enter a file name (and location of the file).The file name is the name of the email attachment, and it is located wherever your email attachments end up --> you should probably set up a permanent location for your PAF database file, and you can move the email attachment there.
Chances are good that the email attachment is a GedCom file (.ged), which can be fully opened from within PAF [FILE menu --> Open --> file type .ged].
Your cousin may have sent you a complete PAF file (.paf), which can also be fully opened from within PAF [FILE menu --> Open --> file type .paf].
There is a possibility your cousin may have sent you a PAF backup file (.zip), which can also be fully opened from within PAF [FILE menu --> restore].