First visit
The shortest honest route from a machine that does not know what an onion address is to the market's storefront. Five steps, in order.
Get Tor Browser from the Tor Project
From torproject.org, the project's own site. Not from a search result and not from a mirror list somebody posted on a forum. Windows, macOS, Linux and Android are all covered by the official builds. When it first opens, it runs a short setup page. Let it finish.
Copy an address from the top of this page
Any of the three. They all open the same market, and none is special. One click on the copy button and the whole string is in your clipboard.
Paste it into the address bar and press enter
Then wait. The first load builds a circuit of about six relays before your request reaches the site. Ten seconds is normal. A full minute on a slow connection is normal too. A blank page during that time is not an error, it is the route being assembled.
Recognize the storefront
When it loads you will see a search bar, a list of vendors, and a login field. That is the market. Nothing on this directory looks like it, and that is correct.
Sign in, or register
Accounts live on the market, not here. If you already have one, use the username and password you created there. If you do not, the registration form sits next to the login field on the storefront. This directory has no accounts of its own and no way to create one.
Two things not to do
Do not open an .onion address in a regular browser. Chrome and Safari cannot resolve the name at all, and the error page you get is not the market being down. It is the address sitting in the wrong browser.
Do not use a clearnet gateway, the .onion.site and .onion.to kind of service, to take a quick look. The gateway sits between you and the market and sees every page you load. If you want to see the market, use Tor Browser. It is the only route where the trip stays yours.