The freetext field lets you add hrml.

i use this field a lot .... 

for some reason it does not paper in the form

If you are talking about field in the form for users, than filtering depends on the text filtering settings set in Joomla for users group, this is a security feature to prevent malicious users from entering javascript etc.

and how do I set the permission, because it's not showing

Filtering is set in global joomla configuration.

and how do I use it, because I added a free text to a form, and nothing appears

Free text is only toadd some text to the form, you add text in field settings. For users to be able to add text you need to use text, tekstarea or WYSWYG editor fields.

I want the user to be able to introduce an html for ads, from specific ad networks

Hi

it is what K said, 

I thought you meant having html text that the form user reads, rather then writes.

And you have to make sure user has ability to write htmll. I believe that is determined in joomla config for usergroups