First of all, it is strongly advised to use Java Runtime Environment 6 or later. However, there is a case where Eclipse does not work with JRE6. In this case you can fallback to running Eclipse in JRE5.
But to make Compose*/Java work properly you need to take some additional steps. JDK6 is only optional when Eclipse is running in JRE6. This is because in that case Compose*/Java can use the Eclipse JDT Java compiler. This compiler is not available when Eclipse is run in JRE5.

Compose*/Java for Eclipse provides an implementation of Compose* to be used with the Java programming language. This release is integrated with the Eclipse development platform.

Requirements:

• Eclipse 3.4/Ganymede with JDT (compatible with 3.5/Galileo)
• Java JRE 5.0 or higher

Optional:

• Java SDK 6.0 or higher when Eclipse runs in JRE 5.0.
  See this page for more information concerning this.

Download the latest Compose*/Java release