You can get the ipod adapter from the dealer, however....... The OEM aux input that's in your center console uses the same jack on the back of the Hu as the ipod adapter. Mazda sells a switching box that will allow you to still have both of these hooked up so you can continue using the 3.5mm jack for something else. If you don't plan on using the 3.5 jack for anything else then that will save you about $120 on the switching box.
There are some other options for ipod integration on the Mz3 however. Look at
My List which is stickied at the top of this section for all the details. All of the ipod integration methods offer the same functions (including the OEM one), except for the audiolink which also gives you a 3.5mm input.
Hope that helps.