How Do I Gain Access To The Content Folder Of A .jar File?
Two questions here so we might need to split off the question into two, but here goes. Jar files are zip, so you can drag them onto the Archive utility or another utility to uncompress them. 

Before you do that, try locating the jar file in finder and then use the right click / control click to open the file. That lets you get a prompt to open non gatekeeper signed files if the Mac is configured with default settings. 

If that doesn’t work, you’ll need help from the administrator to allow you to access that program or have to take it to another computer. Good luck with the Minecraft and mods you might try once you tap into the zip file contents.