XTrkCad uses two directories on your system:
On MS-Windows the Install and Working directories are set when you install XTrkCad.
On Linux the Install directory is usually /usr/local/lib/xtrkcad/, but you can change this by defining the XTRKCADLIB environment variable before starting XTrkCad. The working directory is {HOME}/.xtrkcad/.
Layout files (*.xtc) and export (Section 2.5.1) files (*.xti) can be saved anywhere you wish, as specified in the Save File and Open File dialogs.
Example layouts can be found in the example directory under the XTrkCad install directory.