Hello!
I want to use a custom field I have created in the Task work item in the "MSF for Agile Software Development v5.0" template in the built in reports for this template. I know how to open and edit a report from the template, but I don't know how to find the field I have created.
Although I can't comment on how to find the field exactly in the report editor, I would think you should be able to use the "Ref Name" you've set in the work item editor/XML. I'm guessing you've used your own namespace for this, something like: MyCompany.Common.MyFieldName. You'll also need to set your work item to be Reportable so it is included in the data warehouse of which the reports run. Also note that the data warehouse doesn't get updated instantly, every 2 hours I believe so if you want a quicker update you can rebuild the warehouse manually.
This link explains the different reportable settings well: TFS Reporting Architecture Notes