Microsoft Publisher files – Windows files with the suffix .pub – are simply not compatible with anything other than Publisher, and not even Microsoft Office for the Mac will open them.
It looks like the only free way to get Microsoft Publisher files into something of a usable format for import into other applications on OS X is to export the file out of Publisher as a pdf (on a Windows PC). That way you can then import into Acrobat Pro on OS X – because pdf files are cross-platform – and work with them.
Now, Acrobat can’t completely import the Publisher files and convert them, but the basic text and graphics will be usable. Acrobat Pro is pretty good at editing pdfs and matching fonts in place, so that might be the best way to deal with a .pub file.
If you need to get the files into a page layout program like InDesign, you can import that pdf into InDesign, but you’ll need to redo the layers to a large extent.
Some versions of Publisher allow export in html, so that might be an option, but that depends on what your end product might be. And I haven’t tried that export to see if the images are exported and usable in other applications.
If you have a lot of work in your Publisher files and really need them in InDesign with all the graphics and text retained, there is a third-party plugin for InDesign that will import Publisher files, but it’s not cheap at $199 and I don’t have any experience with it. See Markzware PUB2ID v3 (Convert Publisher to InDesign) Product Page.
Update 6/12/13: Some people have had success using an online document converter called Zamzar to open and convert .pub files.