The GV app doesn't have a setting to change your dialer. GV is technically a forwarding service, it si not a complete phone in itself. You'll have to place calls by having Google call you to call out.
Amy, not true... The latest GV app runs native on Droids, directly dials through Google Voice. App has a setting to use GV for all calls. The Phone App will then do this, BUT only seems to work using contacts to call. Doesn't work using dialer.
Strange. Works fine with my dialer (and has for a very long time). Sorry, I have no suggestions. Have you tried toggling the outbound call settings? Reinstall? Clear cache/data for the GV app? Reboot/battery pull? They're fairly generic things to try.