If the tasks under the project have their percent complete values set (30%, 60%, etc) and the overall project complete is still zero then you probably don't have the task durations set.

If the duration of a task is zero then (to cut a long story short) the calculation is taking the average of zero. If you put one hour as the duration for each of the tasks you should get a valid result. If you have different durations then the overal percentage is weighted for the relative durations.