Both of them should be completed after Priority: Thessia, when you will gain access to Hades Nexus; however you are able to start them earlier.
To add Hades Nexus: Obelisk of Karza quest to your journal, visit the Citadel and the Presidium Commons.
When you get there go to the Courtyard area.
On a bench you should see a bearded researcher holding a Data Pad.
If you approach the researcher you will begin the Hades Nexus: Obelisk of Karza side-quest.
When to start
After Priority: Thessia
“A researcher needs the Obelisk of Karza to help with the Crucible project.
Recover it from the Hades Nexus and deliver it to him in the Presidium Commons.”
Alliance Engineering Corps War Asset Update
To finish Hades Nexus: Obelisk of Karza in ME 3, use the Galaxy Map and locate Hades Nexus Cluster.
Travel to Hades Nexus and then to Hoplos.
Scan the planet named Kopis and send a probe to retrieve the Obelisk of Karza.
Now, return to the Citadel and in the Presidium Commons, find the researcher.
Give him the Obelisk of Karza to complete the mission.
Next side-mission is Silean Nebula: Rings of Alune.