Now that XenServer 5.6 has been released, it allows you to add more supplmental packs during install (so it makes installing OpenManage just as easy as installing the Linux Templates). To get the Supplmental Pack go here, the FTP Link is documented:

http://www.stricklandnetworks.com/blog/2010/07/14/DellAndXenServer-SoHappyTogether.xhtml

To access openmanage after install, goto (just an example) https://myxenservername01:1311
You will have to use XenServer 5.6.