If you are a holder of BNB tokens you get that 50% discount on trading fees.
Looks too late, but who knows.
Not too late. Anyone can buy BNB tokens from the market. If you have the checkbox enabled then those BNBs are used to pay your fees at a 50% discount.
If you don't want to invest in BNB then just wait until you are ready to trade and then buy approximately as many as you will need to pay your expected trading fee.