Fedora 18 uses Grub2 and if you have a grub.conf file in CentOS, it means it is using Grub Legacy. So if you can only boot CentOS and windows that would mean you have the CentOS Grub in the master boot record. Boot into CentOS, login as root user and open a text editor and put the following entry in. You will need to change the root line to the partition for Fedora, The entry below shows the first hard drive, 5th partition Grub Legacy counts drives and partitions from zero:
The #savedefault and #boot lines probably won't be needed. If it fails to boot, delete the hash marks (#) from those two lines and try again. If that fials, go to the site below and read the instructions, download and run the bootinfoscript and post the output, a results.txt file and someone should be able to help.