To post a job, you'll first need to create an open role under the Jobs tab:
From your Jobs tab, you click on the "Create New Job" button:
You'll be taken to the New Job Posting page, where the first thing you should enter is the Job's type, Full-Time, Freelance, or Internship:
The "Name" field is what YOU are going to see when you look at this job on your Jobs page. The "Position Title" is the title of the role you are hiring. Continue adding all the relevant information for the job. The more detail you enter, the better the qualified candidates you will get (check out info on using Skills here).
Be sure to create a Headline that will draw your applicants in on the UnJobBoard. You can find out all about Headlines here.
Once you've entered all the relevant information for your post, you can either save it for later editing or post it to the UnJobBoard.