Hi
Safari Enhancer ought to work if you are using Safari 2.0.4. As Klaus pointed out, it's not compatible with Safari 3.
After downloading the application, did you drag it to your Applications folder before trying to open it? If not, do so, then try opening the application again.
If you are successful, have a look at
this thread for guidance about increasing the Bookmark font size.
Safari Enhancer is useful for a number of functions, including increasing the History file up to 7 days, or 999 entries, changing the color or unread/read links, disabling the Safari cache etc. Make sure you read any developer notes to fully understand what the software offers.
Last point, I use a lot of 3rd party items with Safari. With that said, and adding to Barry's expressed caution in this area, each Safari add-on has the potential to create compatibility problems. If you choose to use these types of items, best to understand first the implications of using any add-on or modifier (the read-me file accompanying most of these items is useful). Also, exercise caution, making only one change to Safari at a time. This way, if a problem quickly crops up on the Safari restart, you'll readily know where to look for resolution.