As I said: The install probably failed.
Users of the Root group don't automatically have access to anything. If you do a check for a specific permission ('spamx.admin', in this case), then the user has to have that right.
During its install, the Spam-X plugin will create a spamx Admin group, give that group the 'spamx.admin' right and then add the Root group as a member of that group. And only then can a Root user access the Spam-X admin page.
Btw, when you reinstall the Spam-X plugin, you will miss the gl_spamx table in the end. That's a bug we've only discovered recently. You will have to create that table manually. The table definition can be found in sql/mysql_tableanddata.php
bye, Dirk