Over-the-air with blackberry browser (need blackberry data plan), or download desktop install files and use desktop manager to install via USB. I don't believe there is any other way.
If you have to install via desktop, then borrow or buy a USB cable. Posted via BlackBerryForums.com Mobile
Note that many/most apps do not have a desktop installation option. You will have to search for ones that do.
Note also, that many apps will not work without a BlackBerry data plan. So before you install them, you might want to check to be sure that they will work without a data plan.