[Plugin: Slideshow Gallery Pro] 403 error

Hi,
After installtion and adding the template code I get 403 forbidden on slide page on the file gallery-css.php even after changing some permissions on files
Any suggestions would be much appreciated
/wp-content/plugins/slideshow-gallery-pro/css/gallery-css.php?v&pID=541&resizeimages=Y&width=960&height=400&thumbheight=75&border=1px+solid+%23CCCCCC&background=%23000000&infobackground=%23999999&infocolor=%23FFFFFF&infomin=Y&ver=3.2.1

https://www.ads-software.com/extend/plugins/slideshow-gallery-pro/