If you have been looking for the Autodesk Seek search window in Revit 2018 and can't find it, you're not alone. Not too long ago, Autodesk Seek was replaced with BIMobjects. Using Revit 2017 or earlier, if you enter a value in the Search field on the "Insert" tab of the ribbon, you will be redirected to the search results on the BIMobject web page, and aside from maybe having to create a new account and seeing a slightly different (and in my opinion, improved) interface, you may not have even noticed that there was anything different going on.
However once you upgrade to Revit 2018, you'll find that the "Search" field has been removed from the "Insert" ribbon tab. Don't panic. There's nothing wrong with your installation, and you can still find the content you used to look for on Autodesk Seek. The intent is for you to install the BIMobject add-in for Revit. To do so, go here:
Once you've downloaded and installed the app, you will have a "BIMobject" ribbon in your Revit session. (BIMobject will install for pretty much every version of Revit that you have installed - or most, anyway. I have Revit 2016, 2017 and 2018 and it created the ribbon in all three).
I won't go into a detailed description of every button on the ribbon, but using the "Browse" button will bring up a dialog box that will allow you to search through the entire content library from within your Revit session. Downloading a family will not only download and save the file automatically to a predetermined location (controllable in the BIMobject settings), but will also bring it into your current Revit project at the same time, ready for placement.
There is one item to be aware of, however. If you choose to take advantage of the more advanced "Properties" functions that are on the ribbon, your Shared Parameter file will be changed to point to a BIMobject Shared Parameter file. For me, personally, this is usually a deal killer, as I consider "Thou shalt not mess with my Shared Parameter file" to be the eleventh commandment. Normally, if I discover that an application has done that, it's going to be uninstalled immediately, but in this case, I'll give BIMobject a pass - as long as you stay away from the "Properties" and "Update Properties" functions, your Shared Parameter file should be safe.