Downloading Atmosphere Jars - Atmosphere/atmosphere GitHub Wiki
To install Atmosphere, the recommended way is to use Maven and just do:
<dependency>
<groupId>org.atmosphere</groupId>
<artifactId>atmosphere-{atmosphere-module}</artifactId>
<version>1.0.x</version>
</dependency>
All the dependencies can also be downloaded from Maven Central
If you aren't using Maven, just download the library from here and unzip it.