You will need to configure your browser to launch the viewing program of your choice. The details of how to do this vary from one browser to another (and sometimes between different versions of the same browser). Examples are given below for two typical cases.
If the file type is not recognised by Internet Explorer, you will be prompted in a popup box to Save or Open the file. Select `Open', and on the following popup box choose the option `Select the program from a list'. You should then locate the copy of the Mercury program in the `Open With...' file browser (see figure below for examples of the dialogue screens). By checking the box marked `Always use the selected program to open this kind of file' you will ensure that the visualization software that you have chosen will in future be associated with CIFs by the operating system.
Select the browser menu `Edit/Preferences', highlight the `Applications' category, and add a new application with the following characteristics:
For other browsers, the steps necessary will be different. Find your browser's help pages on adding helper applications. The crucial point for all browsers is that the `MIME Type' must be specified as chemical/x-cif.