Use an FTP program and log into the directory where the site is hosted.
Right click the files that it says you need CHMOD 777 and set all of the permissions (Owner, Group, World) to Read, Write, and Execute. Do not do this with all of the files, just the files it tell you to or else someone could easily hack your site.
Screenshot of this from my FTP client:
http://www.draggar.net/stuff/chmod777.jpg
CHMOD is a decimal representation of the binary permissions file, first digit is read, second is write, and third is execute.
So, each "set" of permissions is a 3 digit binary code ( 000 though 111) converted to decimal (0-7). 3 set (Owner, group, world) makes it a 3 digit number.
