If you get the error message "not registered" when sending or receiving an MMS it is most probably due to the fact that you have been swapping SIM-cards. The intention was (is!) that it should be possible to swap SIM-cards without affecting anything within SwirlyMMS. Due to a bug in SwirlyMMS versions <= 2.2.5 you might get that error message in some situations after a SIM-card swap.
The workaround is simple, just re-register to SwirlyNet. A new version (2.2.6) is coming soon with a correction to this error.
regards,
The SwirlyTeam