I have tried six different browsers on my Mac and the Place An Ad text shows if I am logged in, does not show if I log out. I got someone else to open a new account on a Mac and as soon as they activated their account it worked OK.
Some of the PC users who cannot see it have been using the other site for years. One sent me a screen snapshot that clearly shows he is logged in but there is no Place An Ad text.
I will try checking permissions but if it works for someone who just opened an account why would it not work for some people who have been using the site for years?
The only other thing I can think of that might help is that at least two of those having this problem have not actually used the site for a long time.