I am having a problem displaying the actual labor units on my bar chart / curve histograms.

I would agree with your last comment. To have actual cost show, actual resource rates need to be tied to the resources. This can be handled in the resources section of primavera. Once you have rates assigned to your project and resources and they incur actual hours, actual costs will start to generate. You can see these in the activities and you can also see these actual costs in the assignments sections. You might be able to see a cumulative in the histogram (if using 8.x) section of resource analysis.