Not sure if it's the issue but your missing &submittedResourceId=`45`, try adding that and changing the id to your page ID.
Also not sure if it could be that your validating last, try moving the &validate to the top of the call, IE:
[[!Register? &submitVar=`registerbtn`
&validate=`nospam:blank, username:required:minLength=^6^, password:required:minLength=^6^, password_confirm:password_confirm=^password^, fullname:required, email:required:email`
&activationResourceId=`153`
&activationEmailTpl=`lgnActivateEmailTpl`
&activationEmailSubject=`Thanks for Registering!`
&usergroups=`userMember`
&successMsg=`Please Check Your Email to Activate the Account`]]
Just guessing though