What is confusing you in the wizard? It should lead you through each step
fairly easily. You tell it that you want the Computers table to be the
results for the query, then you tell it you want the Users table as the one
that contains the related records, then you select the fields in both tables
that would be related/linked (UserID?), then you specify the fields from
Computers table that you want the query to show you, and then you are done.

Otherwise, assuming that the names I've listed above are correct, this is
the SQL statement of the query to show all Computer records containing a
UserID that does not exist in the Users table:

SELECT Computers.*
FROM Computers
LEFT JOIN Users
ON Computers.UserID = Users.UserID
WHERE Users.UserID Is Null;