Data Validation question

I’ve created a drop-down data list in the following way.

1) Selected a cell
2) Clicked 'Data Validation'.
3) Selected 'list' from the drop down 'Allow' menu.
4) Typed my data in the 'Source' box, each separated with a comma.

I now want to edit that list by adding more data, but I don't seem to be able to move along the list to insert data where I want to.
When I press the arrow key to move left or right, it enters a cell ref. instead of moving the cursor along the list.

How do I move along the list so that I can insert the new data where I want to?

Thanks for that, pressing Fn + F2 did the trick.