When you start Entropy, an empty archive document is presented to you. To add files, simply drag the desired files from Finder and drop them on the new archive window. Alternatively, you can also use the "Add" button in Entropy's toolbar to select files for addition.
If you want to remove a file in the archive, select the file by clicking on it and then press the delete key on your keyboard. Alternatively, you can use the "Remove" button in the toolbar.
Once you're done adding files, click on the "Save" toolbar button ( or File > Save in the main menu ). You can select the format of the archive from the drop down in the Save window.
For certain formats, you will be presented with a set of options ( such as compression level, encryption, etc ). You can safely ignore these options if they are not important to you.