To open a file in Squeak, begin by opening a file list. Click on the desktop and select open, then select file list.
Use the upper panes to find your file. Ctrl-click on the file and select fileIn from the menu. You may get a warning about the file, but it shouldn't be a problem. The classes and methods defined in the file you just imported should now show up in the System Browser.
To save your class and method definitions from the System Browser ctrl-click on whatever you want to save and select fileOut. A file with the '.st' extension should show up in your current directory