You have to unpack them, read the readme file..
1) Launch WinZip
2) Drag-drop the .bos file from explorer into the WinZip window.
3) Type Ctrl+A to select all files
4) Press the extract button on the toolbar
5) A dialog should appear. Make sure use folder names is checked. Make sure
the output directory is the core folder inside your tactics install path.
6) Press extract.
If you repeat this process for each of the .bos files mentioned above, you
should now be able to access the tiles and entities and campaign from the
editors.