Sweet Home 3D only checks supported extensions and doesn't analyze the content of files to make them selectable or not in its file chooser.
Maybe you use the Mac App Store version of Sweet Home 3D which is able to import only zipped files because of the sandbox security imposed by Apple. If it's the case, just zip the .dae file (with its textures if there are any) to import your 3D model. In case you didn't know, Mac OS X lets you create easily a .zip archive from the files selected in the Finder with the File > Compress menu item.