Having problem with Url alias

My Drupal is 6.2 and installed at

http://www.sciencenewsheadlines.com

I have problem to add Url alias and it says "page could not be found".

e.g http://www.sciencenewsheadlines.com/health_news

I previously did Url alias without any problems with version 6.1.

Now with version 6.2 I notice " /?q " in the url

e.g.

http://sciencenewsheadlines.com/?q=admin

I think "/?q" might be causing problem with url alias.

How do I correct this?

thanks