Every time I create a flight plan, the Elevation Profile shows the entire flight as a horizontal yellow line fixed at the altitude I set as my cruising altitude (see attached image), instead of an arc representing the true flight plan elevation, with ascent, cruise, descent, etc.  ALSO, the Elevation Profile shows a number of black dotted lines-- possibly representing profiles of other flights (?) -- which I'd like to get rid of somehow! Any tips would be appreciated.

Possibly down to not having a performance file for the aircraft your flying. It takes the info from the performance file like rate of climb and climb speed to work out your flight profile TOC. It takes the decent rate and speed to calculate your TOD.

delete aircraft trial to get rid of black dotted lines. The trail can made to be longer or shorter and also change the colour in the settings.

You can also right click into the elevation profile and select "Delete aircraft trail" to delete the black lines.

You probably see a red error message in the flight plan table footer which explains why the profile cannot be calculated. Luke already mentioned a few reasons above.

Alex