I just programmed my first microcontroller, a attiny13, a couple of days ago. I still have much to learn, but spent the last two days studying the data sheet, and the tn13def file. Don't recall any DIDR. There is a DDR (Data Direction Register), where you set the pins for input or output.
Got me curious, and picked up the printout on tn13def.inc... There is a DIDR0, maybe you forgot the zero on the end. Oh, you never mentioned if you are using assembler or something else, not sure it matters.
Can you post the program? I just started myself, so examples are very useful learning tools. My big problem is setting up the Timer0 as a frequency generator, need a 38kHz output to one of the pins.