If the client machine has the appropriate version of Java installed, there are no additional requirements to use either Java Plug-in or Java Web Start. Netscape Navigator v7 as well as the recent versions of the Mozilla browser include the latest version of Java, while Internet Explorer does not. See the Sun Java System Instant Messaging 7 2006Q4 Release Notes for version requirements.
If the client machine does not have the required version of Java installed, you need to install Java Web Start. You can download and Install Java from http://www.java.sun.com/j2se.
You can download and install Java Web Start from http://www.java.sun.com/products/javawebstart.