Try this:

InStr([Enter Location #:],[Location])>0

If "1,3,9" is text, then InStr() or any method to search text within the field won't work, because searching for "3" would return not just "3" or "1,3,9", but also anything that has a "3" in it: "23", "33", "43", "13,30,1", "1,33,300", etc.

If you want to search for "3" and only get "3", but not "13", "23", "33", "300", etc., then this is not as straightforward as it appears to be.  The key is the *comma*, the delimiter, that separates the different locations, so that "3" wouldn't be confused as "13", "23", etc., because the commas always separate them apart.

You first need to make sure you use only commas as the delimiters in all entries.  Then you separate the different locations with the Split() function.  For instance, Split("1,3,9", ",") returns an *array* containing "1", "3", and "9" individually.

Finally, you compare your lookup value against all the values in the array and see if there is a match.  The VBA code for this is not terribly difficult, but quite involved if you are a beginner in VBA.

If I were in your position, I would not lump all the locations together in one field.  Instead, I would store each location "individually*, in a normalized table such as this:

ProductID   LocationNum
P1000       1
P1000       3
P1000       9
P2000       13
P3000       23
...

Being "normalized" means that things that should be separate, should be.

This would make the query a lot more straightforward, and you probably wouldn't need VBA.  You could have a query that joins your location table with your main product table, with a WHERE clause that could easily find all products that are at location 3:

   SELECT * FROM ProductsT
   INNER JOIN LocationsT ON ProductsT.ProductID = LocationsT.ProductID
   WHERE LocationsT.LocationNum = "3"

I just tested what I posted above and it does work.

The available values in the table: 3, 1, 43, 9, 5, 3, 23

In the parameter dialog I entered: 1,3,9

What was returned: 3, 1, 9, 3

But if you enter "3" as the parameter (which Luis said he was doing), your Instr() function will return 3, 43, and 23, if they are all text.  I assume they are text because Luis said that his location field can contain something like "1,3,9" -- which is a non-normalized way to store this info.