Howto convert geotiff elevation data to esri bil file format?

Use gdal tools, namely gdal_translate[1]. If your geotiff file has spatial specs encoded in it is really easy.

eg;
gdal_translate -of EHdr e020n40.Bathmetry.tif e020n40.bil

The gdal_translate command also creates the necessary bil header files.

[1] http://gdal.org/formats_list.html

Try to view the output with OpenEV.