You must buy the dipstick from the dealer. Also, when setting the level, the fluid must be at 80C. Your dealer has a special device for doing this.
This tranny is very sensitive to the level, so make sure you set it correctly.
You might consider having it done. My dealer charged me an .75 hour's labour plus parts. It was worth the cost for me, but some folks prefer DIY for this job. Given that I was already $200 light for parts, the extra $60 wasn't a big hit...
1998 C230 "Black Betty"