In terminal you would get command not found if the path is incorrect. in this case if you downloaded the 32 bit green download button item at the top of the page you would have to include the path to the application or executable ie /home/user/folder/file.
Although I did not have to do this, If you double click and get a text file, close it and right click the app, choose properties then permissions and see if the check box is checked for make this file executable. if not checked , check it and try again.
If you are using a 64 bit system then you need to download the portable version and run the 64 bit app for linux, it includes win, mac and linux for 32 and 64 bit and includes java.
you will need p7zip to extract the compressed download. it should be in the repository. open the folder where you downloaded the file then right click in the folder and chose open terminal here. in terminal type p7zip -d filename. then look for SweetHome3D-linux x64 and double click.