Infernal Talc (Fiery Dust)

Im in the sewers but there is nothing i can use this item on. What to do in this quest?

In the Warehouse sewers, there's an "Undying Blockage" near the waypoint; usually towards the left.

Where do i get the fiery dust?

The wiki has most of the answers:
http://pathofexile.gamepedia.com/Fiery_Dust

to get the fiery dust, go to the docks and open the green worded chest to get thaumetic sulphite. then take that, along with the ribbon spool you got from the battlefront (you did get that didn't you?) and go to Lady Diala in Solaris Temple level 2. she will give you the fiery dust when you turn in those quests.

you might not need the ribbon spool but it is handy to have with you since you can do both quests at once when you get to her.