Your syntax looks okay to me.
If you open the immediate window you can dynamically test the function by typing the following in;
?=ConcatRelated("OrderDate","Orders","CustomerID=1" )
And see what it returns. Substitute 1 for an actual customer ID.
For what it's worth I too would have pointed out about the cross posting had I spotted it.
Many of us participate on more than one forum, and it's pretty irksome to find that you have spent time on providing an answer, to then get a response with a totally different train of thought involved/code, which has magically appeared from someone else's answer on another forum. We don't mind but it's polite to mention it, as someone may well repeat the advice already given elsewhere.
As you will discover there are normally 3 or 4 ways to skin a cat in Access, and each persons preferred method differs, hence the frustration in the above situation.