When using Java 1.6 beta 2, the GUI apparently tries to download the DTD from the UPPAAL web site. This fails if machine has no network connection.
This is mostly likely caused by Java 1.6 including a stax compatible XML parser, which replaces the one we ship with UPPAAL. Apparently, Sun's parser tries to validate the XML documents.
Fixed on the trunk from rev. 2478.
Fixed on the 4.0 branch from rev. 2479.
The old DTD from UPPAAL 3.4 has been added to the 4.0 branch and the trunk in rev. 2714 and 2715, respectivly. This should solve slow loading of old models when using JDK 1.6.