dsPIC: Using FRC and OSC2 as an I/O pin
I haven't used the part in a while, but it should be just a case of setting the configuration bits correctly. You don't say what compiler you are using so I can't give a code example, but your compiler manual should let you know (the commands will probably be at the end of the include file)  

Also, you are better off reading the 30F family reference manual (Section 7 is the Oscillator) as it's far more detailed than the datasheet.  

If you let us know which compiler and maybe post the code you have already tried it would help give a more specific answer (e.g. a code example)    

I notice in the table below the FPR bits are dashed out for the FRC without PLLoption, even though note 1 says the pin function is dependent on these bits (if so, how should they be set?) This seems suspicious, I think I would raise a support ticket with Microchip to see what they have to say on the matter.