Trying to install linux ubuntu on macbook via virtual box

I am having trouble running ubuntu via virtual box on my macbook... so here is what is happening: i set up the virtual machine with the disk image i want to use then when i go to start installing it, it asks me what language and then it gives me five options: 1) try ubuntu without any change to your computer 2) install ubuntu 3) check disc for defects 3) test memory 5) boot from first hard disk..... no matter which of these i select virtual box freezes and doesn't start installing or doing anything for that matter. I have partitioned a part of my hard drive for the virtual machine to use... i really don't know what to do hoping some one can help me out.