You can explore the area in your own time if you fancy, but to start the main quest, you'll need to rummage around in the skeleton near the start area. Travelling there (and walking forwards a little bit) will transport you to The Wilderness itself, a vast wasteland that feels a lot more abandoned than anything in the Commonwealth.
On installing the mod, players are presented with a fast travel option north of Sanctuary called 'The Wilderness'. Although there are basically no humans, there are plenty of horrible critters to keep you occupied on your travels.
