Syncing is currently development-level code - you need to be able to at least read logfiles! Testing is welcomed; please report to the sync list (sync@lists.horde.org), but please keep in mind that the code is still in heavy development.
The sync URL is http(s)://pathtoyourhordeinstall/rpc.php
Username/pasword: your Horde account data
Databases (sometimes called remote settings) must be calendar, tasks, notes, or contacts.
For debug output create dir /tmp/sync and make it writeable by webserver
Set log level to PEAR_LOG_DEBUG in Horde Setup/Logging. It may be helpful to filter the logs as follows:
tail -f /tmp/horde.log | grep "SyncML:"