Try uninstalling the game, unchecking "show pre-release" in Impulse, and reinstalling 1.05. I recently reverted to 1.05 myself for mod purposes, and the mod button showed up properly this way. Maybe something got goofed up on your patch, who knows.
Impulse actually shows the highest available version, it doesn't show your version (this you can find in-game in the lower right corner on the main menu), so it showed that 1.05 was the highest (before you checked show pre-release), but you were probably still running 1.0 