This plugin is bundled with the IDE and enabled by default. See Manage plugins.
Don't forget to add your libraries to the dependencies of corresponding modules.
Overview of Tiles 3 support
Tiles 3 support in IntelliJ IDEA includes code completion, and syntax and error highlighting for:
Tiles definition files
Expression languages (EL): Unified EL and Object-Graph Navigation Language (OGNL)
Tiles - JSP support tag library