I'm currently updating a Joomla 1.5 site to 2.5. The 1.5 site currently uses Admiror Gallery 3. How do I update to AG 4.5 so all the current galleries still work?

I use this gallery several times with no problems but I've just had something pop up. I created a new gallery and two this happen. I receive this error message: Warning: Division by zero in /var/www/vhosts/bedrockbuilders.com/httpdocs/new/plugins/content/AdmirorGallery/classes/agHelper.php on line 3...

After I install Admiror Gallery 3.0.174 my site no longer works. All the pages display the error message below. I'm using Joomla 1.5. I've used AG before on a different site with no problem. Any suggestions are greatly appreciated. Parse error: syntax error, unexpected T_OBJECT_OPERATOR in /homepage...