Let's say you want to store an integer on MW0.
Inside the plc you can use the MOVE-function or use the result of a calculation.
With WinCC-flexible you can create a tag e.g. 'name_value' or you can make use of the symbolic names of the plc-program but then you need to work integrated between WinCC-flex and STEP7.
Go to your screen and make use of a IO-field (it shoudl be on the right side of the screen).
In the TAB 'General':
Define the 'Type-mode' as input or input/output.
Define the 'process-tag' as 'name_value'.
Define the format-type 'decimal'.
Define the format-pattern according on how big your value is.