Hi Tamsmaillet, sorry to hear you are having trouble with your first Mac. I'll try and help.
It seems Carolyn Samit's instructions weren't detailed enough for you to follow.
First of all, the copy and paste commands are available in every application in the "Edit" menu. You can also use Keyboard commands,
Copy: "Command" key + "C"
Paste: "Command" key + "V"
What Carolyn wanted you to do was select the text, /Library/Preferences/SystemConfiguration and then use the copy command to copy it to the clipboard.
Now go to the Dock at the bottom of the screen and click on "Finder" (far left icon)
Now go to the "Go" menu at the top of the screen and select the "Go to Folder…" command (near the bottom)
In the "Go to the folder" field paste the text you copied above. It should look similar to this.

This will take you to a directory labelled "SystemConfiguration." In that directory find the file called "Networkinterfaces.plist" and drag it to the Trash (far right on the Dock.)
Now restart your computer and try logging back in to your Apple ID in iBooks.