Recently, in a security update Forumotion disabled posts with links for new members, they had also unchecked allowing guests to post.
To allow guest advertising to work again you need to go to General > Settings
Set Prohibit registered members for less than a week to post external links and emails to No
Set Unauthorize guests to post external links and emails to No
this will allow links in ads (and new members posts) to work again!
You also need to go into your Categories and Forums go to Advanced Permissions and ensure Guests have the right to Create A New Topic.
Please spread the word so that others can fix their sites as well!
~Rose.