Search Google on my own script, I came across your problem with my myPh3 script. Very nice to see people actually trying it out and using it! THerefor, I'd be more than willing to help you with your problems.
First of all, you can simply turn off URL rewriting to see it the script is set up correctly. Get your browser to /myPh3/myPh3.core.php (if you installed the script in a website subdirectory /myPh3) hereafter. If this works, we can direct to the rewrite problem. Are you sure your used the /myPh3/myPh3 URL? It does NOT use the directory default page... You can also contact me at the SourceForge site http://myph.sf.net
. Hope to hear from you.