How do you go to slough creek in wolf quest?

In version 2.5 or older, you'll need to save your game after bonding with and securing a mate, then return to the splash/title screen. From there, click 'Single Player' and under continue, click 'Slough Creek'. You'll then be able to load a completed Amethyst Mountain save.

In version 2.7 and newer, after bonding with and securing a mate, a star will appear on your compass and a new zone will appear near the Lamar Overlook wolf territory. Upon entering this zone, you will be prompted to migrate to Slough Creek. Note that you can't migrate from Lost River to Slough Creek. (This would defeat the purpose of the Amethyst Mountain level, hence why the team did not make this possible.)