While most DAWs use the scientific notation, i.e. C4, the middle-C in OpenMPT is C-5 for historic reasons (Impulse Tracker, one of OpenMPT's roots, has an octave range from C-0 to B-9 with C-5 being middle-C).
I can wholeheartedly recommend diving a bit into OpenMPT's manual, which answers a lot of basic questions about the software. For example, the middle-C is explicitly mentioned in the
Pattern editor chapter.
PS: Even when frustrated, please don't open topics with ALL-CAPS titles. This is perceived as shouting and will not make it any quicker to solve your issues.