How to add a stock price?

What you see when you click on the + sign are each lot purchased. No you can not add to this list - other than thru purchases. Sells will remove lots from the list.

I think you are looking for the Price History - right-click on the security name and select Price History. You can add to this list - manually or thru online updates.

Actually, one can modify "historical" prices on the Summary tab of the account or the Portfolio tab of the Investing Center. But op is just not picturing the process correctly: one doesn't really add anything to the lot history for the security (which represents purchase transactions), one just modifies the price/share value shown for a security for a given date. In other words, you chose the date whose price you want to specify with the "As of" box at the top of the pane, then put the cursor in the Quote/Price field for the security (not for the "lot") and key in the price for that date. Once the cursor is moved out of the field, the new price will be visible. This does the same thing as modifying the "Price History" (and for some people or some situations, adding prices to the Price History may be more convenient).

Beautiful! Thank you!

Is the mutual fund a common one that is traded and is represented by a trading symbol? If so, can you simply use the "One Step Update" and let Quicken automagically add the current value for you? Am I missing something in your original Post??