Usually with the new versions o f microdem you either slect the GooglEarth globe in the toolbar or right click on the map and select Export where you are given a choice between quick and deliberate export to Google Earth, however there seems to be a bug in the programme now that doesn't allow adjustment of opacity in Google Earth so you are left with a partially visible srtm and another bug, as you have found, id that it no longer saves as aGeoTiff but as a .tif image.
The only soloution I can suggest at the moment is to find an earlier version of the Microdem programme which appear to work as normal or use QGIS (free full blown geospatial programme)