I started the SoU module the other day and created a Dwarven Fighter, in the hopes of turning him into a Defender when he met the requirements. Well, he's level 5 now and has met all the requirements for a Defender. However, in the level-up screen it's not letting me select the defender, as if I'm still not meeting certain criteria.
Am I not able to make that prestige class in SoU or something?

One of the requirements for Dwarven Defender is a BAB of 7; even with classes that have a high BAB progression (such as a Fighter) you won't have a BAB of 7 until level 7, so level 8 is the soonest you can take any Dwarven Defender levels. Also make sure that the choices you make in game don't shift your character to a neutral or chaotic alignment, as a character needs to be lawful to take any DD levels.

Awesome, thanks for the reply. I took all my equipment off and I still had a BAB of 7. I guess there are some other factors that come into play behind the scenes?

On a completely unrelated note, I'll be playing Hordes of the Underdark after I finish this and I have no idea what type of character to create. I played a human paladin in the base NWN campaign, a sorcerer in BG2, and a dwarven fighter / defender in SoU (obviously). Does anyone have any good ideas for a character build that's different from what I already played? Something to make the experience feel fresh again?
Thanks.

Your strength modifier also adds to the BAB shown on your character sheet (unfortunately I don't think there's anywhere on your character sheet that shows your actual BAB).

As for a new type of character to try, you could give a Shifter build a shot- it'll play a bit differently from a straight-up melee or arcane build like those you've played, as you'll be pulling out a variety of different shapes to deal with different situations. Plus shifters get lots of goodies once they hit epic levels, so HotU is a good campaign to use one on.

You will need a base attack of 7, not BAB and equipment/attribute modifiers don't have any effect to base attack (see more here)

only way to get a base attack of 7 is to get a minimum of 7 levels in some combat class (ie. fighter) or ever more levels in other classes.

you can find your current base attack in character sheet between spell resistance and will saving modifier.

Looks like there's some confusion of terms here...

BAB = BASE attack bonus (increased only through levelling)
AB = Attack bonus (a combination of ability scores, feats, gear and BAB)

The numbers at the top of the character sheet +12/+7/+2 is your AB.

Further down the sheet, you will find your BAB. For a 5th lvl ftr this will be 5. You need to make this 7 by taking at least two more lvls before you can choose a DD.

Have fun :)