Since you talked to the doctor at his house earlier, he will not be there at this point in the game. However, even if you had waited and was able to confront him about the room full of poison, he would just have claimed to have lost the key, so you would be no further ahead than you are now.
Explore the house to continue. Make sure you thoroughly search the cellar, since once you leave with evidence against the doctor you will not be able to get back in.
If you have trouble getting past the front room, hint: <span class='standouttext'>Spoiler : </span><span class='spoiler'>check out the lanterns</span> (highlight the text to read).
