Domain Mapping for Subdirectory install

Okay I just used the domain mapping plugin and couldnt quite get my way around it.

I have installed WP 3.0 with multisite on the root directory. It is a subdirectory install. So http://example.tld is my main install and any sites I create will be http://example.tld/site1/ and so on. Now I want some of these Sites to have their own domain. http://example.tld/site1/ to be http://site1.tld

Now my question is Domain Mapping should be able to do this right?

But apparently I cant achieve it. When I go to Tools->Domain Mapping I only get an option: Custom Domain for Original Blog Address which is http://example.tld and not http://example.tld/site1/ which is what I expect. Is it how it should be or what I am thinking cant be done with your Domain Mapping plugin?

[Links fixed ~ drmike]