First of all, from the TextPad Help menu contents, please see: How to / Use with other applications / The Java Development Kit, and also, How to / Customize settings / User Tool Properties. Please also see the information on the following web site.
http://textpad.com/support/faq/java.html
I have included some other information below, which you may find useful.
To set the CLASSPATH environment variable:
1. Exit from TextPad.
2. Click Start/Settings/Control Panel.
3. Double click System.
4. Click the Advanced tab and then the Environment Variables button.
5. Add CLASSPATH to the list and click OK.
6. Restart TextPad.
Alternatively, you can set it for the Run Java Application tool in TextPad
as follows:
1. From the Configure menu, choose Preferences.
2. Click the "+" next to Tools.
3. Select "Run Java Application".
4. Insert the following text at the start of the Parameters box , just before "$File ":
-classpath path1;path2;etc
where path1, etc. are the paths to your class files.
5. Click OK.
If you are a novice to Java, you may find the following web sites to be useful:
http://www.awoolf.com/javaclass/setup.htm
http://www.joegrip.com/