installing ubuntu on MAC 9,2 in dual boot

Hello
I try to install ubuntu on my MAC book pro. I have a Mac book 9,2 (mid 2012) and try to install ubuntu 12.04.4

I installed reflt and created a 50 GO partition formated in hsf+

I passed successfully the stage of burning the MAC version "ubuntu-12.04.4-desktop-amd64+mac.iso" to a DVD and getting the installation menu. I succeded in getting the menu to format my 50GO with ext4 where I am going to install Ubuntu

But a pop-up appear saying

The partition table format in use on your disks normally requires you to create a separate partition for boot loader code. This partition should be marked for use as a "reserved BIOS boot area" and should be at least 1MB.
If you do not go back to the partitioning menu and correct this error, boot loader installation may fail later, although it may still be possible to install the boot loader to a partition

I have several questions
- is the boot loader code that the pop-up is talking about is corresponding to reflt or anything else ?
- I can create a partition with the disk utility of my MAC and the possible format are
- MAC OS X extended
- MS-DOS (FAT)
- ExtFAT
- free space
or should I create a partition another way (and how ?). In the ubuntu installing menu, I haven't the option to add a partition available

- In any case, I can't see the option "reserved BIOS boot area"
- When the pop-up is talking about boot loader, what is it about ?
- on an eventual new partition, have I to install a software ? If so, which one ?

Thanks in advance for your answers.