your command is incomplete and that's why it's not working it should be
dp.exe import C:\Demodata MicrosoftDynamicsAX
here "dp.exe" is utility name and "Import" is the import command and C:\Demodata is the place where you stored extracted data files and MicrosoftDynamicsAX is Ax2012 Database name .if your database has different name other than default "MicrosoftDynamicsAX" you have to specify that name at the end.
Hope this will help .
Amith Prasanna