As a part of editing Akron (NY) airport, I've noticed that there is a large power converter about one mile from runway 25. I've tried removing it with exclusion rectangles in ADE, but after I compile my airport, I come to find that the rectangle was not saved with the airport. What is causing this?

Not sure what you mean by 'power converter'.?

If you're referring to the power line tower in the approach path to 25, that is not an ordinary scenery object and cannot be removed with an exclusion rectangle. The tower is drawn by the 'Utilities' vectored poly (line) that draws the power line 'greenway' and the towers along it. To remove it......

In FSX, center your ac on the tower.
In ADE, select the (green) Add Polygon tool. Draw a small polygon (rectangle) around your aircraft location. In the Terrain Polygon Properties window select Type=Exclude General and Tag=Utilities.

Click OK and then save and compile the airport. The compile will save  the normal ADE airport bgl and a *CVX.bgl file that contains the 'vectored poly' exclude data.

Note: Due to the way vectored polys are drawn, this type of exclude will remove the utilities' greenway and towers over several km    

Thanks. It worked great. In fact, I think I may be able to upload it in a few days.

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but after I compile my airport, I come to find that the rectangle was not saved with the airport. What is causing this?

I am not sure I understand.  Do you mean it did not work?  Also check in Settings > Options > General Tab whether Separate Airport and Object Bgl Files is checked.  If it is then ADE is putting exclusions and scenery objects into a separate Bgl file with _OBJ in the name and this needs to be placed in the same folder as the airport Bgl file.  For most folks this option need not be checked.