Use this as a starting point
Private Sub CommandButton1_Click()
Cells.Find(What:=TextBox1.Value, After:=ActiveCell, LookIn:=xlFormulas, LookAt _
:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, MatchCase:= _
True).Activate
ActiveCell.Offset(ColumnOffset:=1).Activate
ActiveCell.Formula = ActiveCell.Value + TextBox2.Value
End Sub
If I were you I would use a user Form.
You will need at least two textboxes, one for the ID and one for the amount to add.
I haven't done it hear, but you could also add option buttons so that depending on the selection it will either:
add
subtract
Overwrite to current value.
You also need to write in error handling in case it can't find the stock item.
I wrote a similar thing for a warehouse so as stores personal could look for stock on pallets. I had to include in the application that added new stock to always convert text to uppercase. The reason for this is I also used UPPER in the userform so it is then possible to use match case.