In System 6.0.8 with MacWrite 1.0 how do I open a plain text file?

The relevant metadata isn't actually stored in the resource fork (per Gordon Davisson). The type and creator codes need to be "TEXT" and "MACA" respectively. When importing into Mini vMac with its file importer tool, these will be set to "BINA" and "SITx" which aren't going to let you do anything with it (unless it's an actual Stuffit archive).

Try the Finder Info tool made for this purpose. It opens with a simple window that has text fields for these two pieces of metadata. Change them. There's no need to click a save button, as soon as you have changed them, they take effect (as with most modern Mac app preferences).

The icon for the file should change as well, and it will be openable with MacWrite.