• Double-click on "Macintosh HD"
• Double-click on the Utilities folder
• Double-click on the Virex folder
• Double-click on the Virex icon:

At the top of the window there will be a row of buttons:

 
• Click once on the "e-update" button on the far right.

The computer will go out over the network and download the newest virus definition file from the internet.  This whole process is "invisible" and takes less than 60 seconds!  Once it's finished, go to File, Quit and back to business as usual.