You use the "Stocks, Mutual Funds, Bonds, Other" interview. You tell TurboTax you did not receive a 1099-B. You can use "everything else" as what you sold. Type in an appropriate description, (e.g., Cash Distributions in Excess of Partnership Basis") and a "Date sold" of whatever distribution drove your basis "negative". The "proceeds" would be the amount of "negative basis."
Your basis would be $0 meaning the entire amount of "proceeds/negative basis" would be taxable. Probably the correct date to use for your purchase or acquisition would be the date of your initial contribution, but any date that made the holding "long term" would work, mechanically.
This will be recorded as a Box F sale on Form 8949.
Tom Young
