Triple OS on MacBook Pro8 2 -- missing operating system

Today I am trying to install Windows 7 64bit OS and Ubuntu 12.04 LTS 64bit OS on my Macbook pro (later 2011), I followed this link (http://lifehacker.com/5531037/how-to-triple+boot-your-mac-with-windows-and-linux-no-boot-camp-required) to install these extra two OS.

Everything goes well until I restart the computer to launch Ubuntu through rEFIt. When I restarted my Mac after installing windows and ubuntu, I can see three OS icon and I shift to ubuntu. After a screen flash, it pop up a black screen with one sentence on the top, say, missing operating system.

Initial guess is that Linux GRUB boot loader does not install properly, it might be corrupted by Windows MBR boot loader. Therefore a new boot loader should be created to make all OS can be detectable.

After several searching on google, I found this link (http://ubuntuforums.org/showthread.php?t=1908210) which solved my problem.  As this link addressed the whole installation procedure, given that we have already installed three operating systems, we only need to regenerate a boot loader if something is wrong when we start up computer.  I summarise them as following:

1) Download gdisk from here(http://sourceforge.net/projects/gptfdisk/)
2) Log in Mac OS
3) Open the "Terminal" in Mac OS (lauchpad->utility->terminal, or search from spotlight)
4) Run "sudo gdisk /dev/disk0"
5) Type 'r' and then 'p' to print out the partitions
6) Then type 'h' to create hmbr
7) Type in the partition number corresponding to different OS, for example, #2 for mac, #5 for windows and # 7 for ubuntu, you should type 2 5 7
8) Next select 'y' to place EFI partition first
9) Type in MBR hex code for each partition, usually 'AF' for lion, '07' for Windows and '83' for linux. You might notice that in your machine, the hex code for windows and linux are the same '07', it cause one system is missing which cannot be detected by current boot loader. Note that do not set any of the bootable flag.
10) Finally type 'w' and 'y' to update the MBR

Now when you restart your system, each OS should be working.

Good luck.