While trying to create a separate partition for virtual machines, I realized that I did not change the default CentOS partition layout so it was using LVM. I was a little confused whether to simply just mount the /dev/sdb or work with LVM, so I logged on Freenode and someone told me I need to make it part of the volume group. (I later realized I could've [obviously] just mounted it.) Took me couple hours to read about and understand how LVM works.
I followed these guides, they're very similar:
TLDP LVM HOWTO
CentOS LVM Administration Guide
The guides are a little vague for a newcomer to LVM to I had a hard time, and I got stuck towards the end with how to mount the new space that I had made part of the volume group. I did not want to merge this space into / (root file system) where everything else is, and keep the /vm in a separate partition.
The virtual HDD I had added was now part of the volume group, and I could see it as /dev/VolGroup00/virtmacs (that's the name I gave it). Therefore, for simplicity's sake, I just did mkfs -t ext3 /dev/VolGroup00/virtmacs, and then added it to /etc/fstab and mounted it and took a sigh of relief.
For people who're adding a new hard drive and get confused by Logical Volume Manager, and don't want to deal with it for whatever reason (maybe you're just testing something and don't want to expand storage etc.), just mount as usual. You don't have to go through any special guides.