I bought the diablo 2 and lod cd-rom like 4 years ago...now i decided to play again but i lost my diablo 2 cd key, so i trought if i buy the diablo 2 in blizzard store i could play my expansion again(i have the lod cd key and cd rom). i downloaded and installed the diablo 2 that i bought in blizzard store but when i tried to install my lod it says that i have to install diablo before installing lod but i already installed it...

You need to register that can on your Battle.net Account then. You need to either use both Digital Downloads or both CD's, you can't mix and match.

Ive got the same problem but Im using store bought cds for both the game and expansion....any ideas?

Try just using the Digital Downloads, it makes life a whole lot easier. Here's how to do it:
1) Create and verify a new Blizzard Account, if you don't have one already:
(https://www.blizzard.com/account/).
2) After logging in, click on "Games" under My Account Controls. Or, click here:
(https://www.blizzard.com/account/management/games.xml).
3) Click the Add a New Game button. Or click this:
(https://www.blizzard.com/account/management/add-game.xml).
4) Enter your CD-Key and select your language, when prompted.
5) You can now download the game by clicking the Windows or Apple logos, depending on your OS.
6) Install using the Game Key shown by clicking "[View Game Key]".