Are you using MPLAB or MPLABx (the newer version)?
In general if you create a project and then use the menus to add a new file of the appropriate type (assembler in your case) then it will put in the correct file type.
Is there any reason you are using assembler? While assembler is very powerful and you can use it to get the device to do anything it is capable of, I would not recommend it when you are learning to use PIC24 devices. If you use something like C then you can concentrate on the higher level task of getting the device to do something. If you use assembler then you need to also concentrate on the specifics of the code.
I would suggest that you write something simple (e.g. the 'flash a LED' program) in C and, once it is working, then look at the assembler the compiler creates if you really want to understand how the device works at that level. (This is better if you can use the higher levels of optimisation but generally the free compilers such as XC16 from Microchip only let you use the lower level optimisations.)
Susan