I use plain TeX (not LaTeX) on a Windows 10 PC to create worksheets for my HS math classes. I've been using MiKTeX 2.4 and WinEdt5 (since 2001) but decided to try upgrading to the latest of each (MiKTeX 21.2 and WinEdt 10.3). Where do I put my Macro.tex file so it can be found when compiling? I've tried the directories that make sense, or that are similar to the old system I've been using, but WinEdt/MiKTeX can't find them. Is there a refresh I also need to do? Can anyone help me out?
If you open the MikTeX console, click the "Settings" button, and select the "Directories" tab. This will show you the search path MikTeX is using. You can add your own directories to list, or choose to put your file in one of those directories that MikTeX knows about.
Then, on the "Tasks" tab of the main , window, select "Refresh File Name Database", so that MikTeX becomes aware of the file.
From the comments above, it appears the OP missed this last step. It is a common enough oversight, since it is not needed if replacing an existing file with a newer version...it is only needed if a new file is introduced into the MikTeX directory structure.