Sounds like you really want a DLP device. McAfee sells a network DLP device, though I wouldn't really recommend it. We got it here, and it's not very good. Certainly not up to the level I expect of other McAfee enterprise products. That being said, if you have experience with Snort (and it sounds like you do), then you can write rules using the same syntax for McAfee IPS as it supports Snort signatures.
There are other guides for different versions of NSM, but I don't know how different they are. NSM support can assist you if you encounter problems with the rule creator, but we don't assist with writing the rules .I believe Professional Services is able to assist you in writing specific rules.