there could be alot of reasons why the converter box isn't working. it could be a faulty minidisplay port to vga adapter, faulty vga cable, faulty converter or incorrect converter settings. since you said it shows up fine on a computer monitor i'm assuming the adapter and vga cable works. next check your output resolution on your mac. set it to 640x480 or 800x600. also make sure the red end of the vga cable is connected to the converter. if you are using component make sure the switch is on y cp Cr. and try plugging in cables starting with tv, then the converter and finally the macbook.