If you cannot add in floorplan details and the fields are greyed out, it is most likely due to the invoice date on the stock being in a past accounting period. You will need to do a stock alteration to bring the stock into the current period.
Under the Stock/Sale tab, click Stock, and then Stock Alteration.
Select the stock by clicking the 3 dots, and then change the acquisition date into the current period.
You will now be able to enter floorplan details on the stock.