When you're on the main Upwork site, not the forums (click on the Upwork logo at the top to get to the main site), you should see a menu bar at the top left saying...
JOBS FREELANCERS REPORTS MESSAGES
Just click on JOBS and it will give you the option to post a job or search freelancers.
If you can't see that menu, then you might have a freelancer account and a client account. In that case, you have to go to your username in the top right and use the dropdown menu to switch from your freelancer account to your client account.
If that's not working for any reason, then you're probably best getting in touch with support who I;'m sure will be happy to help.