Loop to add string to bottom of a variable column

Hey guys,

So I have a drop down menu on a worksheet, with values that match up to columns on the same sheet. What I'm trying to do is when a user chooses a value from the dropdown menu and clicks the "run" button, a string (in this case "x") will be placed in the first blank row of the column that corresponds with the value of the dropdown menu.

I've had limited success so far, as I can get the "x" to appear in different places based on the selection in the dropdown menu, just no where close to where I want it to be.

The workbook is attached if anyone could provide some insight or suggestions or point out my mistakes.

Thanks

Here is some of the code as well

Sub test()

Dim str As String
Dim a As Integer
Dim onoff As Integer
Dim c As Integer
With Worksheets("Sheet1")
Dim NextEmptySubCat As Range
Dim rng As String

str = Range("J2").Value
a = 1
onoff = 1

Do Until .Cells(1, a).Value = "" Or onoff = 0
If .Cells(1, a).Value = str Then
rng = a
Set NextEmptySubCat = Range(str & 2).End(xlUp).Offset(0, 1)

NextEmptySubCat.Value = "x"
onoff = 0
Else
 a = a + 1
 End If
Loop

End With




End Sub