It sounds like you have an extra column in your fields tab of the dataset properties.
Try this:
Delete anything extra that you've created, so you're back to your original report. Make sure that runs correctly.
In your report, go to: View > DataSets.
Follow the same steps as before (Adding a calculated field), but this time, set the field expression to =1
Try using that field in your report and see if it runs correctly.
If that works, then change the expression to your calculated measure. If it doesn't, could you please post the error message?
Thanks,
Jessica