Dallas is...
Last in Points Allowed Per Game (29.5)
Last in Interceptions (11)
Second to last in Sacks (19)
Second to last in Total Rushing Yards Allowed (1808)
Last in Total Passing Yards Allowed (2671)
Third to last in Passing TDs Allowed (21)
Last in Rushing TDs Allowed (26)
Last in Takeaways (17)
Last in Turnover Differential (-17)
Second to Last in Rushing Yards Allowed Per Game (120.5)
Last in Pasing Yards Allowed Per Game (178.1)
... and Dallas is hardly the worst rated D in the game. It has a solid defensive line, and has more 90+ men than my Seahawks do! Although your FS is the one weak spot, but your SS is outstanding. It IS a 3-4 defense though, and those can be tricky from what I understand.
Look, it's OK if you aren't good at playing D. Look at my earlier league scores. I simply couldn't stop anyone, and kept getting burned on big plays. I ended up having to figure out a lot of things largely on my own, but once I did I realized that most of Madden is what YOU do(or don't), not what happens to you. As the saying goes, you have to be good to be lucky.
If you need help, just ask(I did and often). Hell, Wellington wrote a guide for 1up, so he should make himself useful there.