If your METAR is supplying current weather data, the icon should be working.
The weather status icon should also be present on the left of the temperature on the main page.
In my case, these icons disappears when my METAR goes offline and reappear when the METAR goes back online.
If not, it could be another issue. You may want to check the belchertown github pages for any open or closed issues and see if someone else has had that problem or post your issue for support on the weewx user group.
Just to clarify - is this a new install and the weather icons never showed up or was it working previously and stopped working?
Your question about the METAR setup brings up another interesting question. Is the METAR station chosen by distance from the location info you put in during the initial setup or can you specify a METAR?
My memory is hazy on this maybe someone else can help. Vince is the resident expert on weewx/belchertown issues.