problems with gtk

Hello everyone,
I just started to discover Darwin Ports, I had used Linux before i  
passed to Mac Os X so i'm excited by the fact that i can use the  
'old' apps i knew.
In particular i'm interested in the use of bluefish.
I installed darwin Ports and then i installed, for example, bzflag;  
everything gone right.
When i try to install bluefish (or cannonsmash, and so everything  
that makes use of gtk ) i receive this kind of error:

powerbook-g4-15-di-mattia-macaluso:~ tia$ bluefish

(bluefish:11670): Gtk-WARNING **: cannot open display:

Then i'm moved to my prompt.
What's the problem? what can i do?
Thank you for the help.
Mattia