Normally all you have to do to open up your items.cfg file is to just right click it with your cursor and click the "open with" option and select which program you want to open it with. It should open with "wordpad" as seen below:
If it isn't opening with wordpad all you have to do is right click it and click "open with" & then "choose a default program". From there you can select the appropriate program to open it with.
Most of the time when you download an item pack all you have to do is replace the new items.cfg file with the old one. Be sure to refer to any "read me" files that could have come with the item pack download because it will tell you whether to replace the old items.cfg file with the new one or to open the new items.cfg file and copy and paste the new codes into the old one of yours.
I hope that clears things up for you.
