Clearwire does not currently offer any modems that have an external antenna port. Coupled with an outdoor directional WiMax antenna, you can greatly improve your connection quality and speed with this modification.
Use a dull flat object to pry the base of the modem off. I used a butter knife and carefully popped loose the tabs holding it together. Be careful not to destroy your modem's case in the process. After the bottom is off, the board should slide right out.