Choose "Other" - the program will still allow you to enter the necessary information, just without assuming you have a 1099-B.
Necessary information is: Description, Date Sold, Proceeds from Sale, Cost Basis and Holding Period (i.e. whatever information is necessary to determine how long you held the stock before selling).
While this was a "stock repurchase" from your employer's point of view, it was the "sale of stock" by you.
