Hi alberto,
We understand the same. once you moved from right side to left side, You can click on Tick(continue mark), next you may get pop like want conform these entries, yes, continue, it may ask for transport request and packages.
continue upto generating extract str. later you can check data sources fields at RSA6, double click onb your data source and see its showing added field or not.
How to do:
From your screen, go to change mode, select MCVBUK-FKSTK(left side pane, yellow in my pic) and click on Choose(arrow mark red mark).
once you moved left to right side, scroll down right side and see, field was added at last row after the blue color fields. next click on green mark, continue.........
Thanks