This is a known defect in TurboTax even though TurboTax does not include it on their lists of "calculations not supported by TurboTax". Certainly non-public, non-QSB stocks can be reported using the installment method.
Try the following. It does seem to work but no guarantees:
Begin the Installment Sales interview under the "Less Common Income" section
Answer "Yes" to the question "Do you want to report an installment sale?"
On the "Property Description" page enter "Description of Property Sold", "Date you Acquired This Property", "Date Your Sold This Property"
On the "Sales Information" page enter "Selling Price", "Sales Expenses" and "Cost or Basis"
On the "How Was This Item Used?" page answer "No"
On the "Sold Stock" page answer "No"
On the page "Collectible" answer "No"
On the page "Passive Activities" answer "No"
On the page "Debt Assumed by Buyer" enter the appropriate amount ($0, I assume)
On the page "Principal and Interest Received" enter the appropriate amounts
On the page "Depreciable Property" answer "No"
On the "Related Party Sale" page answer "No" (I assume)
