I'm 99% sure that Fedora 15 and CentOS use Grub legacy which works fine if you can find the documentation needed. Fedora's Installer has the choice of whether to add existing Linux installs to the bootmenu and I believe by default it does NOT add the existing installations except M.S. You should be able to tell if you are using Grub Legacy by pressing C at your menu prompt to select a O.S to boot. If a command line shell appears it should be grub Legacy.
If it is Legacy and you want to "manually boot" these are the commands.
"kernel" along with a root= argument/option
and then finally "boot" all without quotes.
First command "root (hd0,1)" this would be if you want to boot a Linux O.S that is on your first hard drive on the second partion. Grub Legacy uses 0 for more than a place holder so 0 equals 1 in Grub Legacy talk.2 equals 3 and as well.
root (hd0,1) press enter
For the kernel command you type "kernel /boot/" and then press tab which will give you a list of possible selections.For Fedora type "kernel /boot/v" and then press tab. You should have something like
"kernel /boot/vmlinuz-22.214.171.124-26.rc1.fc15.i686.PAE" you will need the root= option so the kernel command would look like this.
"kernel /boot/vmlinuz-126.96.36.199-26.rc1.fc15.i686.PAE root=/dev/sda2" As you can see the kernel command is different than the root command as in there is no punctuation(comma) in the root= option.If you type sda,2 it will not boot.
The tab "fill in" funtion will work in all commands for your knowledge.
The "initrd" command is easy, you just type "initrd /boot/i" and then "autocomplete" by pressing the tab key.
Lastly you type "boot" and if everything is correct you should see large font boot messages that turn into a small font boot up message scrolling on the screen and then a log in menu should appear. Press "enter" after completing each command along with the boot command.
Thats the manual way to boot. The last Linux O.S you installed has a menu.ls file that you will have to add the O.S that it did not add to your menu.ls file to automatically boot. I think Fedora needs the uuid data to boot in the menu.ls file so you know.
If you get back to your thread members can help you with your unbootable O.S. Multibooting is quite a bit more complicated than dual booting but if you break it down into small steps it is easier to understand.