How can I attach stainless steel utensils to aluminium posts?
You will have better luck brazing the stainless steel (which has nickel and chromium) to a brass or bronze post.  Brazing is a hotter form of soldering,  You might inquire if a jeweler would silver solder it for you.  Other wise, torches, special flux and special solder are needed.

A less expensive option would be a better grade of epoxy.  System 3 or some of the 3M industrial epoxies.  As Chris said, there must be a GOOD MECHANICAL fit first, the part shapes must align closely to allow the adhesive to bind.

On another note, you should try to abrade both parts in the contact area to give the adhesive a "toothing" point to grab.