If you have a Pro or Ultimate plan (or are in a trial) then you can choose to either publish a post to your personal linkedin profile page or to a company page (for which you are an authorised admin on LinkedIn).
When creating a post you will see a toggle option at the very top of the page that will allow you to select either “Me” (which as the name suggests will publish the post to your own personal LinkedIn profile or “Company” which will publish the post to the company page you have selected on the settings page.
If you are an administrator on a company page you should be able to select the page from the drop down menu in the settings page headed “Company Page”.