Here is a tip on how to filter on the Task Subject field when the text needed to filter on is not one of the populated picklist values (including custom text).
The Task Subject field is a combo picklist/text field. By default, it shows the standard Salesforce picklist values for this field but you can add your own value to this list by right-clicking in the conditions box and selecting the "Add Value to List" option. Once selected you will be prompted to type the desired text to base the condition on.
More information on Using Conditions can be found here.
