Create or copy an Xtend file into a Java project, then just open it, this will install the Xtend builder on the project. Now, you will have a problem marker on the first line of code, quick fix it with Ctrl+1, this will add the necessary Xtend libs to the classpath. That's all.
Source: the Getting Started section of Xtend's installation instructions.
Note: as @rzymek says, you will be able to use the overriden operator only in Xtend, Java will only see the method operator_plus
.