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