You need to capture baseline data for creating baseline criteria. There are a couple of ways you can do this. You need to first enable logging in your model.Enable signal logging for signals of interest in your model. More details here: http://www.mathworks.com/help/simulink/ug/exporting-signal-data-using-signal-logging.html
1. Run the simulation to collect the logged data in base workspace. Save the variable in a MAT-file. You can add this MAT-file in the Baseline Criteria section of the test case. Test Manager will identify the signals and will show them as baseline signals. Once added, you can add absolute and relative tolerances to each signal or entire file.
2. If logging is enabled in your model, you can click on "Capture" button in Baseline criteria. You will be asked to specify the location of the MAT-file where you want the baseline data to be captured. Test Manager will then run the simulation and capture the data in the specified MAT-file.
You may also find this link useful: http://www.mathworks.com/help/sltest/examples/creating-baseline-tests.html
