Installing Dirsync is a requirement for enterprise directory synchronisation's. When I say enterprise i mean , directories with over 50,000 active directory objects or users. By using full sql server as apposed to the standard sql express , we can also make the database highly available via SQL clustering.
So to install it requires some small steps.
1. Open a command prompt with elevated privileges and make the directory where the dirsync.exe installation file is located the current directory. Then run this command 'dirsync /fullsql'
2. This will install Dirsync without the standard SQL Express.
3. Next , browse to C:\program files\Microsoft Online Directory Sync\DirSyncInstallShell and follow the install syntax in this Microsoft Article
4. Make Sure you are installing into a minimum of Server 2008R2 SP1 ideally SP2
5. IDFIX is a great utility recently released from Microsoft to troubleshoot DirSync errors which you can download HERE