Two ways to solve "Restricted Access" problem:

A) Database method:
    1) Use phpMyAdmin and in table jos_plugins change value in published column value to 0 for row jstats_activate
    2) uninstall bot_jstats_activate
    3) install and publish mod_jstats_activate

B) File method:
    1) go to directory on Your server (...)/joomla1.5/plugins/system/
    2) replace file bot_jstats_activate.php by empty file with the same name (or remove content of this file)
    3) uninstall bot_jstats_activate
    4) install and publish mod_jstats_activate