kklosson,
SET action is really used for processing/parsing incoming email. That is the example giving in both the User Guide and the Rule Language Reference. It is NOT the right approach for your example.
1- How is the BO that you are trying to change to 'Yes' put into context?
I will assume for a minute that you are storing info about the user that is logged in. In that case your process would be 1 rule:
LoggedInRegularUser.YNattributeName='Yes'
In the case of LoggedInRegularUser, you would not need to indicate a Process Input because LoggedInRegularUser and SystemSettings are always in context in AwareIM. For any other BO, you would need to either indicate the BO as Process Input or use a FIND action, etc to put the BO in context.
Tom