As long as your pass is still valid, you will get the discount. Just show your pass at the train or lift station before boarding to get your discounted ticket. Your pass is valid for a 2-month period (for Eurail Global and Selectpasses) or 1-month (for many 2-country select passes) from the first day you use it. Your pass should have come with a Eurail guide, which explains the discounts and usage, so you might read through that for more clarification. Eurail Passes that include Switzerland will give you a 25 percent discount off trains and lifts above Interlaken (otherwise tickets cost, from Interlaken, about $8 to Lauterbrunnen, $8 to Grindelwald, $15 to Wengen, $200 round-trip to Jungfraujoch; Schilthornbahn cable car otherwise about $100 round-trip from Stechelberg, $75 with early-morning discount).
just curious, If you have used up all your travel days, how are you getting out of the Berner Oberland? Will you be buying a point-to-point ticket to somewhere like Zurich or Geneve to fly out? For example, a ticket from Interlaken Ost to either Zurich or Geneve Airports is about $75. If you are taking a shorter, cheaper train ride somewhere else in your trip, you can buy this ticket and use your pass for the trip out of Switzerland.