I ran into a trouble when installing MagicDraw: Launching installer... Exception in thread "main" java.lang.NoClassDefFoundError: com/zerog/lax/LAX
In most cases such error appears when installer file is truncated or corrupted.
Please try to download an installer again.
Another option is that /usr/bin/java is an old version java. To check which java is used by installer, set environment variable LAX_DEBUG and launch installation again.
By default installer first uses java found in /usr/bin. But there is possibility to use specific VM when launching the installer. Please add parameter LAX_VM to launcher and second parameter path to java executable, here is an example (assuming that java is in /opt/j2sdk1.5.0_03):
./MD_UML_105_unix.sh LAX_VM /opt/j2sdk1.5.0_03/bin/java