The 3D model display is controlled by the Show 3D Bodies setting. To show your 3D models, ensure this option is turned on in the View Configuration panel:
View ► Panels ► View Configuration ► View Options ► Show 3D Bodies
For more information on this area of the software, please review the following page in our documentation: https://www.altium.com/documentation/altium-designer/pcb-pnl-viewconfigurationview-configuration-ad
Additional 3D body view control can be performed on the body in the PCB Panel's 3D Editor section.
In this section, the blue cone controls the visibility of the displayed model. Clicking the cone will toggle through opacity levels, or you can select directly from the drop-down as shown in the image. More information is available here: https://www.altium.com/documentation/altium-designer/pcb-pnl-pcbpcb-3d-models-ad
View ► Panels ► PCB ► 3D Models
The Opacity can also be controlled by the component properties in the Properties panel once the component is selected:
View ► Panels ► Properties