3 ways to open up admin:
counsle: sm_admin
chat public: !admin
chat private: /admin
btw, you gotta use admins_simple.ini for configuration, have a look:
http://wiki.alliedmods.net/Adding_Admins_(SourceMod)
also, you dont need to add the plugins to any list. sourcemod automatically detects and runs plugins. you can use "sm plugins list" in counsle to list all of the admins and so forth (sm plugins will give you a list of commands you can do)
p.s. the plugin manager plugin is very useful for loading/unloading/reloading plugins on the fly!