Weird. This didnt happen to me, I can do it fine on the localhost right now....
Ill try some more then.
EDIT: Im sorry I cant reproduce this.
Its working fine on my 3.0.0 on localhost, and I copied the installer code directly, overwriting the code I had there.
You can try to edit the actiontemplate array in the beginning, change
PHP Code:
'modifyrpg' => array(
'modifyrpg'
),
to
PHP Code:
'updaterpg' => array(
'modifyrpg'
),
Thats really all I can find remotely cause of this error.