you followed the instructions for installing jack yes ?
there is a front end for starting/using jack called qjackctl
you will need to install realtime-lsm kernel module and follow its instructions
(if you already have it do "modprobe realtime any=1")
run jack as root (this will give you realtime and mem locking which is a good thing)
this might not be a bad way to start out as root using sudo or however
(just run qjackctl as root and there you go)
now the other issue is rosegarden -- i think since it's KDE it can't run as root.
i'm not trying to knock a certain project and just because i find rosegarden confusing doesn't really mean squat -- it might be great.
but you might want to check out muse
its works good and can run as root till you get going enough to feel like rebuilding your kernel for realtime support for users.
if you have more problems just remember to include exact error messages
if you want to run jack without qjackctl as root use
jackd -R -dalsa -dhw:0 -r48000 -p1024 -n2