Struts 2 is relatively young, so the IDE plugin makers have yet to come up with a good one. At least I don't know of one. If someone else does, please let us know.
Besides, who says you need and IDE made specifically for Struts 2? Just import the strut2-blank-2.0.x.war file that comes with the download into either Eclipse or Net Beans and you hvae a ready-made Struts 2 project. Both Net Beans and Eclipse have tools that will help you with the XML files required by Struts 2 (validation, ctrl-space prompting, etc.). They will also help you deploy the application to a server. If you're using Eclipse, just make sure you downloaded the "Java EE" version of Eclipse.