Session["MemberType"]
That will be null as nothing has been stored in the MemberType Session variable. You'll have to check for nulls before using the variable.
if (Session["MemberType"] != null && Session["MemberType"].ToString() == "Admin")
Note the page_load code runs before your button click even does so maybe you want this code inside the button click event. It also won't stop people navigating directly to the adminpanel if they know the page url. Consider using the begin request event in the global.asa to check the user's rights in a single place for all pages, or use the aquire authorisation event however I don't think you can access the Session in that event.