Following Watson's instructions, you will click on "Members List" which will take you to a page with lots of pages which will have a bar with the following:
Quiltingboard Forums: Members List Search Members 
I just posted and clicked on the "Search Members" from within this post and it takes you to the
"Advanced Search" page, so will behave as indicated below.
If you click on the pointing down arrow, next to the "
Search Members," there is a text box that you can type the name (or even part of the name) in or there is also an option to select "Advanced Search" The "
Advanced Search" will bring up another page.
If you start typing the name there will be some of the options that you can click on, but then you will have to scroll to the bottom of the page to click on the "Search Now" box.
If you know the whole name then you can just type it in the text box. If you only know part of the name, then it may give pages and pages, depending on how common the part name is.
If it is someone who you have private messaged with before, you may be able to click on their username to take them to their user profile.
If it is someone who has posted in a bookmarked or subscribed thread, you can click on their name from the thread,
Hope at least something in this has helped.
Janey - Neat people never make the exciting discoveries I do.