Creating a Target List using a Condition of the CRM On Demand Contact's Task/Activity

Hello.  Our company plans on executing a Campaign from ODM based on an event that has taken place over the course of 3 days.

Since our focus is more on Contacts than Leads in CRM On Demand, we have been updating the Task/Activity Record on the Contact profile with registration information from this event.

Now, we would like to execute a campaign from ODM targeting only those Contacts that have a particulur Task/Activity associated to them by the Subject.

When I was in ODM building my Target List for this Campaign, it doesn't appear in the List Conditions I have choice to filter on Task/Activity for the CRM OD Contact? I will need to filter on this type of condition to target specific Contacts for this Event.  Again, these Contacts have specific Tasks/Activity associated to them by the Event.