Macro for transforming 2 way matrix into 1 way matrix
Your description is not very clear, but I see the usual suspects.

Indenting

Always indent your code properly - this makes it easier to read, easier to pick out the logic loops and easier to maintain. In itself, it does not guarantee code correctness.

Option Explicit

Always put Option Explicit at the top of your modules.

Variable names

Use descriptive variable names. For what you tell us the code is intended to do, curDate and curCountry mean nothing. Neither does colsX, rowsX, def, Rng or Mrange.

Use arrays

Using arrays will improve your performance significantly. The reason for this improvement is that you stop switching between the VBA Engine and the Excel Engine (which comes at a cost) and you work purely in the one context. You also stop working with complex objects, which also incur an overhead.

The following is an example of how to adjust the code, but is not intended to be an accurate representation of your code.

Dim originalMatrix as Variant
originalMatrix = Selection.Value '&lt;-- creates an array from the range values - only time to touch Excel
Dim title as String
title = CStr(originalMatrix(LBound(originalMatrix,1), LBound(originalMatrix,2))
Dim finalMatrix(,) As Variant '&lt;-- or whatever type you want here
ReDim finalmatrix(UBound(originalMatrix,1) * UBound(originalMatrix,2),2)

' Adjust following loops to pick the right rows and columns.
For iterator1 = LBound(originalMatrix,1) To UBound(originalMatrix,1)
    For iterator2 = LBound(originalMatrix,2) To UBound(originalMatrix,2)
        finalMatrix(someCounter,1) = someValue1
        finalMatrix(someCounter,2) = someValue2
    Next iterator2
Next iterator1

'.... some code here to select output range and resize to match finalMatrix …
' … and then
outputRange.Value = finalMatrix '&lt;--- again, a light touch against the Excel model


The use of LBound and UBound protect you against any confusion as to whether your array is 1-based or 0-based.You could store these in variables if you wanted the code to read a bit better.