you where close with the last query, you just need to group the OR clauses using perens:
SELECT MemberID AS MEMID FROM tblmembers
WHERE (tblmembers.MemberID = 1
OR tblmembers.MemberID = 6
OR tblmembers.MemberID = 4
OR tblmembers.MemberID = 5
OR tblmembers.MemberID = 13
OR tblmembers.MemberID = 2)
AND tblmembers.memberid NOT IN (SELECT MemberID FROM tbleventmember WHERE EventID = 15);
or you could use:
SELECT MemberID AS MEMID FROM tblmembers
WHERE tblmembers.MemberID IN(1 ,6 ,4 ,5 ,13 ,2)
AND tblmembers.memberid NOT IN (SELECT MemberID FROM tbleventmember WHERE EventID = 15);