Are any of the connectors working for you? If not, you may be running across a bug that failed to populate the OJS database with some information that's necessary for the Citation Markup Assistant to function. This was fixed in 2.3.7, but if you've had an unusual upgrade path, you may be experiencing the same issue...http://pkp.sfu.ca/bugzilla/show_bug.cgi?id=6817
If this isn't it, it's possible that CrossRef has changed their API. We're probably not going to be updating the existing Citation Markup Assistant, as we've replaced much of the functionality within a newer XML parsing pipeline (coming soon). CrossRef is on our radar here but we haven't actually gotten far along enough for me to know whether the API has changed in a way that'd cause trouble. I'll investigate if I get a chance.