"at"

[Peter Lesterhuis]

I want to start an app at a certain time, say 2 hours from now. For 
instance xmms, oowriter or mplayer. I can't make this happen when I use 
"at".
When I type a shell-command like "cp" using "at", then it works.
In the documentation I cannot find any limitations of launching apps 
with "at". Is it an authorization problem (launching x-based apps?).
On the other hand mplayer won't start either.
I'm aware this is not a typical question for this list, but I don know 
where else to put my question.
Thanks in advance.
Peter Lesterhuis