I'm from Germany and some things seem to be different here (can also depend on my style...).
We encourage our students to attend seminars when they finished their beginners course (approx. 6 - 7 weeks). I also did. My first big seminar was confusing, but that's normal, I think.
I was not jet really on a seminar outside an affiliated dojo, but I will go. And I don't ask the permission of my sensei every time. We spoke about this earlier and he agreed, so it's no question any more. And, agreeing with Chuck, change your sensei when he forbids you to visit others!
I visited many dojos, even outside my affiliation (and outside Germany). I always asked their permission before visiting (but without any introduction of my sensei) and it never was a problem. When I started this, I was 1st kyu and I, from my experience, also recomment a solid base in your "home"-style. Then you can enjoy and take to your advantage the great variety Aikido has to offer.
Hope you find the way working best for you,