Partition Problem NEED HELP FAST!!

I have Western Digital external hard drive that I use to store regular files and I just took some files off my friends PC. I ran the Western Digital Toolkit and it said it was going to setup a partition so it can be viewed on the PC. I got the files and went back to my Mac and it's booting off the PC partition and only showing those files. How do I get my HD to boot of the other partition. I know it wasn't reformatted or the files deleted because it took like 5 seconds for the partition to be created and thus work. Reformatting would have took a long time on a 150gig HD.

Does anyone know how I can get back to the Mac partition?