A Behaviour defines different lists of actions and thus their 
availability in different places. Custom Actions do currently not 
show up in the Context Menu tab, but this is actually an oversight 
and I'll change this with one of the next updates.

A work around would be to define the action you want to place in the 
menu as Action in the Action Factory of the Behaviour Construction 
Kit and ticking the "Global action" checkbox for it. It will then 
show up under "Actions > Miscellaneous" in the Context Menu Item 
Library.

Best regards, 
Felix