I take it, you already have seen
this article on how to get started with Substance LAF.
To add the substance.jar to your classpath from eclipse
1) Create new eclipse project for which you are planning to use the substance LAF
2) Right click project->Properties
3) Java Build Path->Libraries tab
4) Click add external jars and navigate and point to the substance.jar saved on your disk
5) If you have saved the jar in your project itself, click add jars instead and follow the same process.