Domain Mapping not working?

Hey All,
I decided to test out the domain mapping plugin offered here beofre going live with my site.

I have changed the A record in the DNS for one of my domains (for testing) but I just get the sever default page for that domain now and the admin area does not work for that blog either.

I have Media Temple (dv) server the MU install works flawlessly, any tips or advice?