Answer:
Search around for a cave north of the Winterhold college. An imperial called Septimus Signus will give you a dwarven sphere and a dwarven cube.
You need to enter a Dwemer ruin and find a table-like device at the bottom. Your quest tracker will point you towards Alftand, but you might have already found one at Mzinchaleft or Raldbthar. The dwarven sphere will open a staircase leading down to an exit to Blackreach.
Follow your quest tracker to find the Tower of Mzark. At the top of this is the device the Dwemer used to transcribe the Elder Scroll. Place the Lexicon on the mounting, and press the third button until the lexicon opens up, the second button until the first button lights up, and the first to complete the process. Take the lexicon (which is part of Septimus' quest) and the Elder Scroll from the machine (which is needed for the main quest).