I'll have to find a new provider if multicast still isn't supported. I'm completely dependent on it for service discovery.

For Hazelcast and other data grid solutions multicast is much more preferable to tcp as it allows dynamic cluster node discovery, with tcp you need to supply a list of well know addresses which is hard without fixed ip addresses.
I've seen people using Zookeeper to work around this but it is extra faff.

Elastic Search needs multicast in order to autodiscover nodes in the cluster.

Multicast is also needed to use CARP (Common Address Redundancy Protocol) with FreeBSD … or of course Linux.

I want to use digital ocean as a backup cloud environment, but need multi-casting support at the private network level, for clustering of servers. Without this ability I cannot cluster.

Is there any instructions, to enable multicast manually?

Multicast is almost mandatory in multinode environments (for instance sessions replication in java apps servers) and in some common caches replicators as Terracotta.

I think it does not Support. I just checked with them