To view a list of companies, you need to run a search. The best ways to do this are:
1. Use the global search box from the top right, and select "Companies"
2. Hover over "Companies" in the main navigation > Show All. This brings a list of every Company you have added. To refine your search, click the "Filter List" button from the taskbar.

You can select as many filter options as you need, and then click "Run Filter" to search again.
Save Filter allows you to quickly re-run this search from the main Company navigation menu. This page will explain more on how to create a bookmark.
Once run, your search results will appear in the main window.
If the company you are looking for is not visible in the list, try clicking "Show Inactive" from the taskbar - since it may have been set inactive (but not deleted).
You can also export the results into a spreadsheet.