To import a Model Asset into Modest3D, a project must be created. So after creating your project select the project you want to import assets into.
While in the select project page, notice the option reading Import Models.
This will open the Import Models tab, click on the Import button to open a folder on your machine where you have model assets stored
Navigate through your file folder to find the desired model asset, select the model asset
After selecting your model asset Modest3D will import the model in to use in the editor.
To Import Image/Audio in the projects option page select the Files tab and in the files tab hit Upload to open a folder on your machine where you have Image/Audio assets stored
Navigate through the stored Image/Audio files select the desired assets
After selecting the Image/Audio asset Modest3D will import the asset ready for use in the editor