Actually you dont even need to "build" it unless you're putting other objects on it, just convert it and put the .shp and either the .htx or the .bmp files in the texsrc folder.
If you gave the tile you built the same name as the converted .ac file then the actual object would have been overwritten creating an invalid file. You prolly need to clear out your shplib folder and convert the .ac file again. I've also found that ground objects look better if you make their elevation .001 instead of 0.