If you are setting up forecasts or budgets on a project you will create forecast models where the details are associated for reporting. If you need to create a original model, then you should ccess this from the project area page\setup\forecasts. Otherwise, if you do the view details option from the project parameters, the form hides the type field so you won’t be able to set it.
Here is a quick overview.
AX2012 R3 CU11.