QTM's requirements
QTM requires Qt v4.4; this is available through online archives in any recent version of Linux. The Windows version is supplied with the latest stable version of Qt in the package. (Qt v4.4 is available in the backports archive for Ubuntu Hardy; I am not sure if I can use Launchpad to issue packages for Hardy, but you can build from source if you enable the "unsupported updates" on Hardy.)
Markdown support requires an external Markdown program, which is usually a Perl script. On the Mac, the script installs into the bundle and Perl is provided by the operating system; on Linux, Perl is also provided and you can choose to install Markdown or not at compile time (note: on Ubuntu and Debian, you can just install the "markdown" package with APT). On Windows, the installer package includes Markdown, but you will need to install Perl and tell QTM where to find it. You can download Perl from Strawberry Perl, and then you need to tell QTM where to find it by going to Preferences (under the File menu), clicking the box marked "Use Markdown in Preview", then clicking the tab "Fonts & Markdown", and entering the following in the "Path to Perl" field (clear the existing contents): "c:/strawberry/perl/bin/perl.exe".
Some older Linux distributions still use the old version of KDE (version 3) which relies on Qt 3 (unlike KDE 4, supplied with modern distributions, which is based on Qt 4). Qt 3 is not sufficient to run QTM; Qt 3 and Qt 4 are neither source nor binary compatible. However, QTM will run quite happily on KDE 3 and 4 and GNOME or any other Unix desktop, and if you use the system tray icon, it will display correctly in both KDE and GNOME, whose system trays are both written to FreeDesktop.org specifications.

