question about the simple guest book

sorry for i don't really know how to install this guest book. i think i did what readme.txt said. but every time when i do the sign the guestbook, it shows 'Guestbook - Error
Cannot create ID file guest_id.txt in /home/usr/you/book. Please check your base directory.'

but i checked the book folder, there is a file call guest_id.txt there already

can i ask what should i do ????? help!!!

thanks.