Hi Simone and congratulations on buying your new Dob! I have the same telescope except that it came with a 26mm 2" eyepiece: I seriously doubt that you need an extension tube to use a 30mm eyepiece that was supplied with the scope. I would certainly phone Bintel; it sounds like something is going wrong when you are inserting the 2" eyepiece. Wait until you get a look at the moon and a few planets, I'm sure you will be more than pleased with your purchase!
