To download the Twitch app go to feed-the-beast.com and click download now
If you have downloaded the Twitch app from the Feed the Beast website please skip to step 4
Step 1
Click the Mods
button at the top of the screen

Step 2
Click Minecraft

Step 3
Click Enable Minecraft
and on the popup click Yes, Enable Minecraft


Step 4
When Twitch has restarted click Install

Step 5
Click the Browse FTB Modpacks
and browse for your desired pack

Step 6
Click Install
on the pack you want to download and wait for it to download

FTB Launcher For linux users or as a last resort for Windows/MacOS users