Half fare card and super saver tickets

Having the HFC only becomes an issue when the conductor comes by to check your tickets. You have to show them the ticket and your valid pass.

No, you don't have to have the Half Fare Card when you buy the ticket. There is no number to enter. Just have the Half Fare Card bought and in your possession when you board the train. If you get checked with a Half Fare ticket and no Card, it is the same as having no ticket at all.

Supersaver tickets have two prices, on with the Half Fare Card, and one without.
If you opt for the Half Fare price, you don't need the card when you buy, only when you are on the train. When the ticket inspector checks your ticket on the train, you have to show both the card and supersaver ticket.
So you can buy the Half Fare Card when you arrive, at any border station or airport station at the manned ticket desk; or you can buy in advance on the SBB website.

Just to clarify, super saver tickets and half fare tickets are two different things.

The Half Fare Card entitles you to half price tickets on just about anything that moves in Switzerland, whether it's a local tram or the ride up to the Jungfraujoch. As said above, you don't need the card to buy the tickets at half price - just select Half/Children's Fare from the ticket machine, or choose Half Fare when buying online. You do need the card in your possession when actually riding.

Super saver tickets are tickets, bought in advance, for which discounts are being offered. These can be bought by anybody, but if you have a Half Fare Card, you get them at half price (so, a discount on the discount!).

Thanks everyone for your info. I won't be going until Sept., but I have been checking for 30 days out now, and it looks as if there might be some available on my route. Of course, I know that many things can change by then.