The following worked for me:
1) Open MapPoint
2) Open the Drawing Toolbar - go to View\Toolbar\Drawing
3) Click on the radius circle and click on a city on the map.
4) Expand the radius to the desired distance.
5) Click Data\Territories\Create Manually. Click Next.
6) Select Zip Code and click Next.
7) Click Finish.
Within the radius circle select Zip Codes needed and add them to the
territory.
9) When they are all added, select the box from the Navigation toolbar.
10)Draw a box around the outside of the radius circle. It doesn't matter
how big the box is as long as it covers the entire circle and all Zip
Codes that have been selected.
11)Click Data\Export to Excel. A spreadsheet is created called Custom
Territories. Click on it, save and rename it as needed.