@ lavieenrose. I´ll try to break it down.
Changing filepermissions at my webhost is done by ftp, when I log in to my account by ftp i´ll see every folder and file that is in my space. Now I dont now your webhost, but if you have the ability to log in to your account via a ftp program like filezilla or something equal. Then you see folders and files that are in your account.
The path to wordpress if installed in your root is /public_html/
The path to the folder with nextgen gallery could be something like
Now if you are able to locate this folder, then you should see wich permissions that are set for that folder, look for numbers like 710, 755, 777 on the right side of folder name. If you use filezilla like I do then you just right click on the folder and choose file attribut and then you can decide permission for the entire folder and make sure that you choose to change permission for all sub folders and files at the same time. After that let the program do the work.
This is how I have to do when changing permission on a file or folder, but it may not look the same for you.
In short: log in by ftp look for the pluginfolder and look for numbers 710, 755, 777 somewhere on the same row.
Hope it helps you a little bit.
Im off to work so it might take awhile to respond.