Geotag is a self-signed Java application. This just means that I'm not willing to pay a lot of money for a commercial certificate I can use to sign a free application. The latest versions of Java 7 (update 51 and later) won't allow self-signed applications by default. You will see an error message like this:

You will have to tell Java to make an exception for Geotag. You have to open the Java security settings panel and add both http://geotag.sourceforge.net and http://kent.dl.sourceforge.net to the exception site listlike this:

After you have done this, Java will still nag you about security risks, but you should be able to run Geotag. To get more detailed instructions for this procedure have a look here and here.