Not all DVD players, particularly older ones, accept some home-made DVDs. Have you tried with different DVDs?
Could be poor media: most of us here recommend Verbatim DVD-R, burned at only 2x or 4x.
If you successfully burned the DVD with iDVD and it plays on your Mac, it is unlikely you did something wrong, but full instructions are here:
A good guide to iDVD 7 can be found here:
http://www.kenstone.net/fcp_homepage/idvd_07_stone.html
Apple's tutorial on iDVD 07/08 is here:
http://manuals.info.apple.com/en/idvd_08_getting_started.pdf
Previewing before burning:
http://support.apple.com/kb/PH5447