Language selection dialog
You need to have a Java Virtual Machine 1.4 or greater installed on your system. If you don't have that, take it from http://java.sun.com.
After Java is installed, simply click the i18nedit.jar file or issue the command java -jar i18nedit.jar.
If you start I18NEdit for the first time, a dialog will appear which asks you about the language of I18NEdit's own user interface. It defaults to your platform's locale. You can change this setting also lateron.
Language selection dialog
I18NEdit should come up. Before you start, do some small initial steps. Select "Settings > Name & Initials". Enter your full name and check your initials. Perhaps set them to something more unique, as the initials will distinct you from other co-workers on the projects you are working on. Finally, click on "Save".
If you want to work with I18NEdit itself in another language, you can select "Settings > Localisation for I18NEdit itself" menu. You have to restart the program for the changes to take effect.
Now, you want to open a project. You do this by "Projects > Open project". A file selection box appears. Go into the root directory of the project and simply select any file. This will open the project at this place. Note that I18NEdit reminds your last opened projects and presents them also in the "Projects" menu. You can reopen a project just by selecting it out of that list.
Open a project