My computer does not have the swing.properties. How to make a swing.properties file and with what software?

notepad? it's basically just a textfile which you give the extension .properties

Ok, so I made it with my text editor and wrote this:
swing.defaultlaf=javax.swing.plaf.nimbus.NimbusLookAndFeel
expecting Nimbus. I still get Metal. Why?

I have no idea. how are you using that properties file in your code?

another way to specify the current L&F is to use the swing.properties file to set the swing.defaultlaf property. This file, which you may need to create, is located in the lib directory of Sun's Java release (other vendors of Java may use a different location). For example, if you're using the Java interpreter in javaHomeDirectory\bin, then the swing.properties file (if it exists) is in javaHomeDirectory\lib

http://docs.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html#properties