Do you want to Run a Script, and you want to know how to access the
Outlook object model, or do you really want Custom Actions? You can
do a lot of functions with VB Script, you don't necessarily need to
use Custom Actions. As noted on the Slipstick site, Custom Actions
require C++, and there is a bug limiting their wide use - this isn't
the case with scripts.
There are excellent books available about the object model, and CDO
and Outlook Redemption can be used to get where the standard tools
can't.
HTH