You may have to hit the "update permalinks" button in your permalinks options if you've overwritten the existing htaccess file. Or maybe you need to create a new page. I know squat about that kind of stuff apart from adding a few redirects.
For the record, what I did in my case was copy and rename the htaccess file on the server, download it, add my redirects, rename it to .htaccess again, and upload it by
ftp. That preserved all the WordPress htaccess stuff.
Everything worked just fine, except for one time I'm missed a space in a redirect rule and the whole site 500'd. That was a scary moment.