interfFrancesco Ammannati,
Assuming this is a MS-DOS program that you are attempting to extend the available memory to you can right click the shortcut to the MS-DOS program and click properties. In the Properties window there should be a Memory tab. Within the Memory tab you can increase the Intial memory environment and the Extended Memory or (EMS).
Note: If this is not a MS-DOS program you will not have the option to adjust the memory.