Can anyone tell me how to point Sweet Home to a file on my USB drive using a Linux OS?

When you insert the USB drive a new drive appears in your filemanager. Notice the drive name. When you open a file in Sweet Home 3D you go to that drive to open the file. The drive is most likely located in the /media directory. In Sweet Home 3D first select the top directory /, then media, then your USB drive.

Another way is to use your filemanager to go to the USB drive and to the directory where the file is located. Right click the file and choose "Open with". Select the option "Custom command" and browse to the directory where Sweet Home 3D is installed. Select the script "SweetHome3D-Java3D-1_5_2" to start Sweet Home 3D with your project file.