You can't stop the stock OS from blocking third party apps. You can, however, root your device or create a CM7 card that will allow you to install these apps.
I have no idea what that means.
Here is a very good explanation of what rooting and CM7 are and their differences:
In a nutshell, if your are attached to the stock interface, you'll want to root. If you want to preserve your warranty, you'll want to go the CM7 sd card route. You can always purchase a ready made sd card from either http://www.rootmynookcolor.com/ or http://n2acards.com/.
Neither process is really that difficult.
Read it but didn't understand it at all. I'm basically retarded when it comes to computers - only browser I can figure out how to use is ie and was never able to figure out how to use winzip or winrar.