I think this topic needs to be moved to the TestDirector/Quality Center part of the forum.
Anyway, there are 2 ways to do this:
1. go to the execution grid of the testlab and fill in the columns for Planned Exec Time and Planned Exec Date
2. go to the execution flow of the testlab and click on the clock icon (most right one in my case). It will appear in the flowchart and when you double click on it you can set the date and time. Then you need to link the clock with the tests you want to schedule (you can do that by drawing a link from the clock to the test with your mouse)