You can participate in the Greenify beta program by joining their Google+ community, which should work with Xposed for Lollipop. As part of the beta program, you'll automatically receive beta updates through the Play Store.
Otherwise, you can grab the APK from this mirror and install it yourself.
Someone notes that it still works with this workaround
[You] can click outside the popup message at start of boot manager and it works regardless of the "module not enabled" message.
Also note that as of today (4/Mar/15) Xposed for Lollipop is still an alpha version. It is not yet complete, and therefore several modules may not yet work with it. Even when it is finalized, it may take more time for the modules themselves to be updated with support.
The latest status update (Feb 19) to the Official XDA Forum also states:
Keep in mind that this isn't my fulltime job and that an alpha phase might take some time. It would be illusionary to assume that we reach a stable state after a few days, with all the changes that have been done." source
The most you can do is contact the module developers and support them in the process of updating their apps for Xposed Lollipop support.