Bring the Toxic Fogger to the Venture Co. sky port, then return to Apothecary Zamah.
The toxin is ready. All that remains is its delivery. You, [player], must make that delivery.
The Venture Company must have a means of sending messages and light cargo between its headquarters and its operations in Stonetalon. It is likely a small sky port -- not big enough for a zeppelin, but perhaps for a flying machine. You must search the areas in Stonetalon under control of the Venture Company and find this sky port, if it exists. If you find it, then place the toxin there.
Place it, and run.