You should be able to use the plug under the hood to reset the service light without any problems.
If you are in the US, then your car is OBDII -- all cars beginning with model year 1996 are. I don't know when the standard was adopted in Europe, etc. It may have been in 1999 or maybe 2000 -- I am not certain.
The two ports you are talking about are completely different things. OBDII is a standard that car manufacturers have agreed upon (the one under your steering wheel). The round connector is a BMW propietary connector that is used for several things, including resetting you service lights, and I believe diagnostics also.
About the ABS light: that is usually a dealer thing. That they found nothing is kind or weird. Did you check your safety belt clips? I hear those being faulty can fire your BAS light. The thing is that I don't know of any tool available in the aftermarket to read your ABS diagnostics. Furthermore, I seriously doubt that buying the equipment would be more economical than having somebody fix it.
Oh -- and welcome to the board