I've tested this feature once, but I remember that has seemed  rather tricky to me, but working.
In order to use this feature, you need to manually open a new ticket  and set it as a master first, then link the others ticket you wish to control to the master, which should be visible in the link list, now. 
As far as I can remember there's no way to use an existing ticket as a master: you need to open one only for this purpose.
Anyway, is more complex to explain than to make it, believe me: sorry for the confused explanation, d. 

________________________________________
Da: otrs-bounces@otrs.org [otrs-bounces@otrs.org] per conto di Leonardo Certuche [leonardo.certuche@itcon-ltda.com]
Inviato: venerd́ 5 marzo 2010 15.18
A: User questions and discussions about OTRS.
Oggetto: [otrs] unable to use master/slave feature

Hi there,

We have OTRS 2.4.5 with ITSM 1.3.2 and just installed http://ftp.otrs.org/pub/otrs/packages/MasterSlave-1.0.1.opm in order to take advantage of its features (announced athttp://otrs.org/2.4/features/)

I installed the package, reload the web interface and even restarted the OTRS service but I still don't see the master or slave options when linking a ticket (I only see parent and child options)

I'm assuming this options are shown at /otrs/index.pl?Action=AgentLinkObject&SourceObject=Ticket<http://index.pl?Action=AgentLinkObject&SourceObject=Ticket> is that right? Is there something else I should do in order have this feature working?