As a merchant, I can tell you that you are far better off using references like this
:Runtime:textures:MyTextures:mytexturemap.jpg
than using the Windoze convention of slashes in the file names. It will prevent all sorts of problems as far as providing support for your customers. I think it may even be a requirement, from memory.
And no, the merchant utility doesn't have any tools to change the path names. You'll have to use a decent text editor with search and replace, as has been mentioned. Once you do get a grip on how Poser's mental file system works, it does become easier, believe me.

P.S. Mac people do buy Poser products, so it's considered good form to take note of the Mac compatibility issues. :)