(Migrating from Java-Selenium to C#-Selenium)
When searching for explicit waits with Selenium and C# I find several posts with code that looks simil
Have you tried adding?
using OpenQA.Selenium.Support.UI;
That should be all you need. Depending on your IDE, it likely (?) will have a helper that will find and add missing references. I use Visual Studio. I just type what I know should be there. When the IDE puts red squiggles on WebDriverWait, I hover over it and it shows a tooltip that says I'm missing a reference and I click on the link, Show potential fixes. In the new tooltip it shows several options, usually the first of which is to add the using statement. I click that one and the red squiggles go away.
I really like Visual Studio but I've been using Eclipse for the last 9 months or so and I prefer the way Eclipse handles this. I think it's more intuitive... but that's just me.
EDIT
Just wanted to make clear for future readers... I don't use NuGet. I just download the Selenium .Net package and drop the 4 DLLs in/near my project and then add references to all 4 of them to my Visual Studio project. Once I do that and use the instructions above, it works for me.