Dwemer Lift is a small Dwemer ruin found along the road to Fort Valus. When following Lurius Liore toWyrmstooth Barrow, the road will split in the mountains before you reach the steampools, the road to the right will lead you past the Dwemer Lift to Fort Valus.
The Dwemer Lift leads directly to the dragon's den in Dimfrost and will remain closed until you've encountered the dragon and used the lift to escape. You can take the lift back down down into Dimfrost once the dragon has been defeated.
Note that if the player has not activated the lift from Dimfrost then the quickest way to return to Dimfrost is through the Wyrmstooth Crypt! So, the Lift must be activated as soon as the player discovers it in Dimfrost.