Not sure why the variable does not appear in the drop down menu. As far as I remember, the list should display the vars that have been passed to the report suite for the last month.
I would advise to not wait, just type in the variable name in the field and hit Enter. It will allow you to configure the processing rules right away.
I figure it out myself. You need to enable Activity map for that report suite to show the context data in processing rule (However, I am not able to see any real data right now)
Can some of you add this to the document in the processing rule, as none of the material mention this and it is pure misleading!