Make sure you've used the reposted binaries of the 5.1.0 install (EPO510L2.ZIP) as McAfee addressed some compatiblity issues with the original RTW build.
Has your server pulled down the latest product compatibility list? You may need to get it to do that to not block the DLP extension install. If your server is ring-fenced and do not have Internet access, try this (citing http://kb.mcafee.com/corporate/index?page=content&id=KB78420):
Use the following steps to manually update the Product Compatibility List:
NOTE: This process is also documented in a Help page in the ePO 5.0 console. 1.Log on to the ePO 5.0 console.
2.Click Menu, Configuration, Server Settings.
3.Click Product Compatibility List in the Setting Categories column.
NOTE: The default setting is Enabled. However, if you set this to Disabled, the XML file can be uploaded directly into ePO 5.0 by browsing on your local file system. The file link is located on that same page. The direct link to the online Product Compatibility List is located at: https://epo.mcafee.com/ProductCompatibilityList.xml.