For troubleshooting on Linux, see FreeMind on Linux.
Not all issues that you encounter are a result of a bug in FreeMind. There may be a problem with your Java installation.
Java installation: You may need to check you Java installation, by entering a DOS console, and typing "java -version" to see whether Java is correctly installed and in what Java version.
Looking at error log: The first place to look at when you suspect a FreeMind bug is the error log. With FreeMind 0.8.0, run FreeMind from freemind.bat to see all the logged errors in the DOS console. With FreeMind 0.9.0 beta, the error log is a text file located at ".freemind" subfolder of your home folder; the name of the error log file is one of "log.0", "log.1", etc.
Java problems: To find out whether your problem is caused by Java virtual machine, you may try another Java Swing application, such as JEdit. If your problem is also shown by JEdit, it is more likely to be a Java one. Trying out the Eclipse development environment does not help, as Eclipse, although a Java application, is not a Swing application.