Tokens and extending rule support

i don't think I understand the creation/extension of rules correctly.

What I am trying to do is:

for an existing trigger/when (a forum reply is created), create a condition/if of:
Forum Reply: Author
with a custom token User: Number of Forum Replies
whose value is the result of a piece of code defined in my plugin.
and the existing action/then "award acheivement".

What I'm not getting is the magic to connect my ITokensRegistrar class to the rules engine condition/if IConfigurableRuleCondition. I've been following what's at and but I think I'm missing something fundamental to connect the custom code to the Forum Reply: Author "type". Any pointers?