The technician can be found to the right of the Purgatory entrance looking at a datapad in her left hand. Listen to her conversation to acquire the mission.
Note: This mission must be completed before completing Priority: Tuchanka as it will be the last opportunity for the player to visit the Citadel before Priority: The Citadel II, after which it will no longer be available.
The schematics can be acquired examining the Old Grid Schematics on the console in front of the controls for the cannon during the N7: Cerberus Attack mission on Tuchanka. Take them before turning on the power to the cannons. If you didn't retrieve them from the mission you can buy it from Spectre Requisitions in the Citadel Embassies for 1,000 credits. Return the item to Alison the technician, who is still standing outside Purgatory.