How can i create a field for the post, that have a few categories?

[seriiserii825]

I use the version 3.0.
I write
->where( 'post_term', '=', array( 'field' => 'slug', 'value' => 'ambulance', 'taxonomy' => 'category', ) )
and i get a error.
After i clear, and write
->show_on_category('ambulance')
and it works, but why?

That this setting is for version 1.6.

And my main question.
How to create a field for the post, that have multiple categories?
Thanks.

And please, give me the link to the docs, where you find solution.
Thanks again.