In order to @tag a user in your post, use the user's Instagram handle. You can get the handle from the profile page - for Nike in the example below, it'd be @nike.

To tag a user in the caption, type "@", followed by their Instagram handle. You can always verify a tag by clicking on it in the editor and ensuring it links to the correct profile.

You can also tag users in the videos / images by going to the Post Options section and selecting "+ Tag Users".

From there, click anywhere on the image (or use the input field if it's a video) to enter the Instagram handle of any user(s) you want to tag.