When you first open the file, any marker that is not a default MDF marker (or PLB, if you chose that template) will not have any settings configured. All of these will not parse, and that will make it look like other fields don't parse, when in fact they do. So the first step is to configure *some* settings for them.
Click on the red ??? to open the dialog for configuring a marker. The minimum thing to do is to assign it a parent marker. If you know where it fits in the hierarchy (e.g., entry level or sense level), go ahead and fill in the appropriate parent (e.g., lx or sn). Look for the "Parent Markers" portion of the dialog, and the word "(New)". Select that, and type the marker that this one comes under (without the backslash).
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Assign *what seems to be true of this data*, even if it is not what you ultimately want for import. The goal at this point is to get as many markers to parse as possible, discover the structure of the database, and discover things that need to be fixed.
If possible, choose "Once" for how many times it can occur. In general, the more restrictive you are, the more you will learn about the structure of the file. If you know for sure that it can occur more than once under its parent, choose "One or more times 'together'". Only choose "One or more times" if you are certain that it may be interspersed with other markers (or after you have worked with the data enough to discover that).
There may be some that you truly don't know, and it is okay to leave some unassigned at this point, but try to assign as many as possible.