Either install an a2sd script, such as DarkTremor or the variant linked IIRC from the first post of the CM7 XDA thread, or download an app called Simple2Ext from the market.
If you use DarkTremor, after installing the script open a terminal and type
su
a2sd reinstall
The phone will reboot with apps moved. To move the dalvik cache with DT use the command "a2sd cachesd" in the same way.