Go into Numbers->Preferences and turn off "use header names as labels".
The references are correct. With "use header names as labels", if there is a column that is uniquely named in your spreadsheet (no other column in any other table has the same name in the header), that's all it needs in the formula to reference that column.
On a side note, you are using MAXIFS incorrectly. The second parameter (the 'criteria') needs to be a range. MONTH(FXUSD::Date) is a single value. It will give you the month of the value in FXUSD::Date that is in the same row as the formula. So if your formula is in row 5 of whatever table it is in, FXUSD::Date is referring to cell FXUSD::A5. It is a little more complicated than that but that's the gist of it. You will either need a column in FXUSD that calculates the months of all those dates or use two criteria.
=MAXIFS(FXUSD::B,FXUSD::A,">="&somedate, FXUSD::A,"<="&EOMONTH(somedate,0))
where somedate is a date
This will work for all dates in a given month of a single year but not all dates in a given month of all years.