Click on the "Games" button in the toolbar of the main IGS window to display a list of all currently ongoing games. To observe a game, double-click on a line, or right-click to open a popup menu which also allows to start observing. You can watch several games at the same time.
The games list can be sorted by clicking on the column headers in the table.
To unobserve a game, simply close the board window.
When observing multiple games, the stone sounds coming from several boards might get annoying. You can turn sound off for individual boards by clicking on the sound toggle button in the boards toolbar.