Right now (during the Indigo 5 beta), you have to manually put the plugin into the
/Library/Application Support/Perceptive Automation/Indigo 5/Plugins folder. That's where all (enabled) plugins live. (If you disable them, they move to
Disabled Plugins.)
I believe the plan is that by the time Indigo 5 is released, you can just double-click the plugin anywhere and it will get moved into the right spot (kind of like what happens to widgets). But that's just what I hear.
Cheers
-- perry