Not to worry. The Slitherine Member search isn't as simple to use unfortunately. First, you need to go to the below site and log in:
Then once you have logged in, you can either click the "Member List" button in the top right. You cannot search for specific users unfortunately, but you can limit the search by name and browse for this. Then you can use the User Control Panel to search
However there is an easier way. Once you are logged in, you will see an option for Use Control Panel in the top left. Clicking this will show a number of options, and you can either message a specific user (where you can search for their username) and you can add "friends", which allows you a quick link to view the person's profile.
For posting, it works in a similar way. Once you've logged in, you'll see "Post New Thread" buttons on the primary forum pages, and if you are looking at a specific thread you will see "Post Reply" buttons. I hope this helps, but let me know if you want any further clarification, especially if you have trouble with the user control panel, and I will try to get some screenshots to show you the pathing you can take and to show how to nagivate the panel.