The estimated audience field can appear as "unknown" for several reasons.
1. You have selected "Send To Individual Number" but have not typed in the number yet.
2. You have selected "Send To Selected Groups" but have not selected your group yet.
3. You have selected "Send To Entire Database" but have not hit the refresh button beside the "Estimate of Selected Audience" field.
Please always remeber to click the refresh button after you perform one of these above actions.