If your having trouble reading commercial dvds i.e. films then you need to install libdvdcss which is available for download as a tarball
or as an rpm/deb package
- just click on these links to go to the appropriate download pages and install them
I hope this sorts you out - I haven't installed this as yet on my laptop but I will be shortly.
Any further problems try ogles' web site
as they have a faq section there and other resources for users.