I am following the directions for downloading a extension, but I am not able to paste into the text bar that I am instructed to download it to.
Here is the page for the extension that I want to download and the page I found it:
https://inkscape.org/~Moini/%E2%98%85realscale-resize-by-line-of-known-length 

Where it states for Installation:

Copy the files realscale.py and realscale.inx into the directory indicated in Edit -> Preferences -> System: User extensions

When I open the Edit>Preferences etc. I find a text bar that when I try to paste the files the "Paste" is greyed out to prevent me from using the paste function

Any advice?

The Preferences dialog simply indicates the directory on your machine where you need to manually paste the files.

Thanks, Tyler. 
I am not interpreting clearly what you mean. By 
"your machine" I should look in the File Manager?  I keep reading in the directions that in Inkscape, I should open the Inkscape App,
then follow Edit > Preferences > System: User extensions. There paste the files I was given and told to paste somewhere. 
Can you clarify what you mean by "your machine", please?

Describe your OS & version, the version of Inkscape and the installer you used.

Windows 10 x64
Inkscape 0.91

Installed via Windows File Manager (?)
 

ok,

So the location indicated in the Preferences dialog will be something like:  C:/Users/[your username]/AppData/Roaming/Inkscape/Extensions.

Use windows file manager/explorer to paste the two files (*.py and *.inx) into that location.

Example: