After you click the Compile button in the DSL Compiler dialog, Lingvo will analyze your old dictionary and display the results in the About Dictionary dialog:
In this dialog you can change the name of the dictionary, its source language and its target language.
When compiling a new .dsl dictionary you will see the same About Dictionary dialog telling you that the dictionary has been successfully compiled. The dictionary parameters displayed in this dialog cannot be altered because they are explicitly specified in the source DSL code.
For details see:
User dictionaries created in previous versions of Lingvo