If you're going to run a Hexxit server and a client on the same machine, make sure you have at least 4GB of RAM. Just to be safe. If you're just running the hexxit client, 2 or 3GB is enough.
Select "Hexxit" from the dropdown menu on the top left.
You can also select the build version of Hexxit from the "Options" button under "Login", in case the server is using a developer version.
You'll now be at the standard Minecraft screen. You can play Hexxit on your own in singleplayer mode, but it was designed with multiplayer servers in mind.
If you receive the message "Update failed" while installing Hexxit, go to options in the Technic launcher and clear your cache. Then try logging in again.
If it then tells you that clearing the cache failed, delete the Hexxit folder in the .techniclauncher folder. Try clearing the cache and logging in again.
Hexxit for Linux
On Linux once downloaded the Linux version of the launcher:
Right click the file technic-launcher-latest.jar, go to properties then on the open window go to the tab permissions and check the box allowing execution of the file as a program.
Then right click if java is already installed click open with java
If Java isn't installed right click and got to open with other applications. Then click find applications online, it will prompt you to select various java applications choose which one, that is currently stable at the time.
Once a Java application is downloaded choose it to run the Technic launcher.
Choose your mod pack and enter your Minecraft username + password and sign into Minecraft.