Hi Julia,
From the drop down you need to pick Custom.
Prior to that you need to write .Net code using our sample solution that we provide with the product to reset the session.
Please consult our documentation (section Custom Actions) about how to write/deploy a custom action.
If you need more info about session check this link
http://msdn.microsoft.com/en-us/library/ms178581.aspx
There are other treads on this forum where custom actions were discussed. Please search for them to get more information.