problems uploading sketch.arduino uno

Dear All, i'm new in forum, i received 2 days ago arduino uno, after installing software i try the example "blink" with success.
i try to modify the value of delay 5-6 times with successful ulploading and i see the led blinking diferently.

then i try ti open new example but when i uload it i receive error

avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_recv(): programmer is not responding

I try to reset arduino several times as describe in trubleshooting, but it gives the same error.
i try also to re-install program and driver
i have powerbook with osx 10.4.11
can you give me ny suggestion?

Search the forums. 10.4 causes some issues (especially if you are connected through a hub.)

http://arduino.cc/forum/index.php/topic,55405.0.html