If you received the objective to "Help Sorine with her project to  improve the crossbow", go speak with Sorine Jurard on the first level of Fort Dawnguard. Ask her what you can do to help. Sorine tells you that she wants to add some punch to the crossbow capabilities. She has done all she can on her own, and now needs a little inspiration. Sorine has always been fascinated with Dwemer artifacts. she takes them apart, pokes around in their guts, and tries to figure things out - with mixed results. But as luck would have it, she has tracked down the location of some actual Dwemer diagram drawings! She wants you to collect those plans for her.

This begins the quest:

Ancient Technology
Sorine Jurard has sent me to retireve the Enhanced Crossbow Schematic from Halted Stream Camp. She will be able to use it to master new technology to provide to the Dawnguard members.

Retrieve the Enhanced Crossbow Schematic from Halted Stream Camp

NOTE: The location of the schematic will be random. It could be some Dwarven Ruins such as Kagrenzel, or a different bandit hideout such as Redoran's Retreat. You can do this quest six times, and each time get a different location. The items you need to retrieve can be different too and might include Dwemer Exploding Shock Bolt Schematic, Dwemer Exploding Fire Bolt Schematic, Dwemer Exploding Ice Bolt Schematic, and Enhanced Dwarven Crossbow Schematics.

Fast travel to the location of the camp/ruins if you have been there already, or to the closes point to it. You will potentially find vampires there. Enter the dungeon. Fight your way through to the end of the dungeon, and find the Enhanced Crossbow Schematic in a chest.

Turn the Schematics in to Sorine Jurard at Fort Dawnguard. You get the Schematics as a reward, as well as are able to buy from her the new upgrades. This completes the quest, "Ancient Technology".