Nikon's D300 is not yet supported in the Camera Raw Lab. If you look several posts down in the X4 forum, there is a list of supported cameras. http://coreldraw.com/forums/t/3740.aspx

Doubtless, Corel will be releasing updates for the Raw Lab to keep up with Camera Model Releases. I am uncertain of the frequency of these releases at this point.

For a workaround you can either usen Nikon's software which came with your camera to create a Tiff or download the big A's DNG converter and use it. I would expect from the discussions heard that Nikon and Canon Models will recieve pretty prompt attention.

Just to let everyone know (apologies if you do already) D300 nef files are importable into Photopaint X4 when X4 service pack is installed.

Although I could not find the d300 on the list on the  web site, they do work now.

We might not yet have had the time to update the knowledge base article... I posted the list of supported cameras with SP1 in my blog a few weeks back: http://coreldraw.com/blogs/gerard/archive/2008/05/12/corel-photo-paint-x4-camera-raw-support-with-service-pack-1.aspx and the D300 is on the list.

Gérard