Install Fiji on MacOSX
First, go to Fiji's main page:
Click on the Downloads link to get at the downloads page:
Download the MacOSX package (and make sure that you read the MacOSX tips). When asked, you can choose to open it right away:
To be able to update Fiji, you should drag the Fiji.app icon onto the Desktop:
Now you can double click on the icon and Fiji starts.