Textures are automatically displayed on a mesh if they are connected to the correct channels. If textures are not displayed in the viewport, check if the 'Material Editor' displays the file path of the textures when you select a material.
If textures are not imported correctly, they can be reassigned by dragging a texture into a texture slot or by using the (browse) icon.
It is recommended to export FBX files with 'Embed Media' enabled to ensures that textures are packed inside the FBX. This ensures correct import behaviour inside of InstaLOD Studio XL.