Luis,

On this page you will find hundreds of models for SH3D.
Click on the link beneath the image, and save the file.
Normally the file will be saved to the Downloads directory, unless you have specified another location.
Open SH3D, click Furniture -> Import furniture -> Choose model.
Now go to your Downloads directory and search for the model you saved. If there are many files in that directory, use *.zip or *.obj to find SH3D models.
Click on the model and follow the steps to import it.

Hans