It should work typing in the Search Field. What is different is that, while the search returns pictures with the keyword,

it also returns pictures with text of the keyword or location names, and any images that it thinks might be the search item. But it doesn't tell which ones are from keywords. It does indicate by icon how it found some results. In the above example, there is a text icon next to the first one, but those are the ones with keywords.
There are items, like Dollhouse for me, that it doesn't recognize, and I get pictures with the keyword Dollhouse.

A Smart Album tells me that I have 10 pictures with "Dollhouse" as a keyword. The search says "indexing" probably because it had not already included "Dollhouse" in its database of objects or text. It's the first time I've used the Search filed to hunt for "Dollhouse;" I wonder if the "indexing" will be replace by other things in the future.
If it's still not working for you, try these things:
- Restart the computer (of course)
- Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this:
Start up your Mac in safe mode - Apple Support
Those are the easiest things to try first. Then try these:
- Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. If you use iCloud Photos, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.
- Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon. Keywords should work right off, though searches for objects, faces, locations and the like will have to wait until Photos scans the pictures records it in the database.
- Make a new user and see if the same problem recurs with it. This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder, or make a new Library.
The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.
Let us know what happens…