Melchsee-Frutt

That's a resort in the mountains above Meiringen which is beyond the Brienz end of the Brienzersee east of Interlaken and cross country from the Brunig Pass.

It appears to me that the nearest train would be on the Zentralbahn which connects Luzern and Interlaken Ost via Meiringen, probably at either Meiringen or up near the Brunig Pass.

The Zentralbahn is reachable via Basel-Bern-Spiez-Interlaken Ost or Basel-Olten-Zurich-Luzern and both of those routes and the Zentralbahn are covered by Select Passes if they include Switzerland.

How you get up to Melchsee-Frutt, I don't know. I didn't see any indication of Postbus, and I can't work out where the network of cable cars feeds from in the summer.

Have you asked your lodging how to get there? Which station did they say was nearest and easiest?

I did notice that virtually all the lifts and trails are still closed at the moment.

I've done a bit more digging.

It is now clear to me that the way to get there is Basel-Zurich-Luzern-Zentralbahn to Sarnen-then bus to Stoeckalp then cablecar up the last bit (fastest) or Basel-Bern-Spiez-Interlaken Ost- then Zentralbahn to Sarnen and up (most scenic).

Sorry I don't know if the bus from Sarnen is a Postbus and covered or another.

Trains to Sarnen are covered by Eurail passes, but a bus connection won't be.