When installing Diablo 2 expansion: Lords of Destruction, I get a screen telling me: "You must install Diablo II before installing Diablo II: Lords of Destruction." As I have already installed Diablo 2 I don't really understand why it won't allow me to install the expansion. I didn't find any help so I decided I'd make a forums post if someone has had the same problem. However I did find some post saying that digital and retail versions are incompatible but this shouldn't be the problem as both of my Diablo's are retail.
Did you get a new Diablo II or Lord of Destruction disc recently? Those have been remastered to use the new installer so that applies to them as well. If you want to, you can just register all the games on a Battle.net account (http://www.battle.net) and download the new installers for both games.