I have search all over the web and have never seen what causes this error Fatal error: Call to undefined function get_header(). When I run wp online it runs fine but running it on dream weaver it creates the error. I know someone is going to suggest to check with Dream Weaver but it is more than them. I was able to use wp with dream weaver before I upgraded to the lastest version of wp so please check to see if something is broken in the new release.
Before you say there is nothing wrong do a search on the net and you'll see how many people are having a problem with this with no answer.