HOW can i render a animation with a alphachannel (transparent Background) ?

1. Render to image sequence, using an image format that supports Poser's alpha channel (PNG, TIFF, PSD).

2. Render to AVI, using a video codec that supports Poser's alpha channel (Huffyuv, Lagarith). You'll need to configure both codecs to "Always suggest RGB for output", and with Huffy, at least, you'll need to enable the RGBA option.

Where can I find good BVH motion capture files for Poser 6?

You'll find a wide variety for sale here in the Marketplace, at DAZ 3D, and at Credo Interactive.

As far as free mo-cap is concerned, your mileage may vary. Most of these weren't designed specifically for Poser figures, and will usually require some tweaking after import:

http://acabar.cafwap.net/~zier/anim/
http://www.e-motek.com/entertainment/index.htm
http://www.mastudios.com/motion.html
http://www.help3d.com/
http://www.cs.wisc.edu/~pingelm/ComputerAnimation/bvh.html
http://www.ict.usc.edu/graphics/animWeb/humanoid/sampleData.html
http://www.bvhfiles.com/
http://www.hypervision.co.uk/

And the free 30MB archive from Santa Monica Studios, containing over 200 .bvh files:

http://movement.stanford.edu/santa.zip