To get a pack, visit here for the information you need and where to send it.
You will need to create an XML document to start building your pack. An explanation of this process is available on the XML Guide page. Login to the admin area and then go to the Versions for your pack, then edit the dev version. Here you can paste your XML for your modpack.
You can also navigate to the Files section to upload your mods.
The easiest way to get started is to set up the base pack version information (pack) then to setup your loader (loaders) and then to upload all your mods for the pack to your files section, and then select them all and click the XML button to get the xml for all the mods selected. This should get you 90% of the way to making a pack. From here you can tweak and play around with the XML.
To see your modpack in the launcher you'll need to add your Minecraft username as a tester from the Settings page.
You'll also need to fill in any other fields in the Settings page for your pack, such as pack image and description.
Once you've built your modpack you can publish your pack. This will make it visible on the launcher.
We have a one time per account approval step where ATLauncher staff need to check over your pack's xml, and make sure it all looks good before you can publish.