I just tried it, and it worked for me. Perhaps the site was down (temporarirly).
There is one (known) issue: The SDK has been released at 2.4.0 UIMA-AS (and its Eclipse plugin) have not yet been released at 2.4.0, but are still at 2.3.1.
The deployment descriptor editor (part of UIMA-AS) (at version 2.3.1) depends on the base component descriptor editor at the same version level. Therefore, if you want to install all of the Eclipse plugs for UIMA, you must install the 2.3.1 versions. If you install the 2.4.0 version of the uima runtime and tooling, by itself, that will install OK, but then you won't be able to install the UIMA-AS Deployment Descriptor Editor.