From what I've read on the BPAL forum, you need a lot of perfume oil for a reed diffuser. Like a couple of 5ml bottles. As for what you would mix it with, that would be dipropylene glycol (which seems to be something you can get from a soapmaking supply place). One set of directions on a soapmaking supply site I found said that you should use a 50/50 mix of oil and dg, so if you had a four-ounce diffuser, you should use two ounces of perfume oil and two ounces of dg.
A better option might be a room spray. That's much easier. Distilled water and the oil. Shake before use. You can also add some Everclear or 98% rubbing alcohol (or, hell, perfumers' alcohol, if you happen to have any, although I wouldn't run out and buy some for this without trying cheaper/easier-to-get alcohols first) to act as a preservative. Added bonus: It can also double as a linen spray, although you should be careful about staining if that's a concern. The proportions depend on how strong you want it, but a good place to start is take a four-ounce sprayer, add a shot or so of alcohol and a sample of perfume oil, and fill the rest of the bottle with distilled water.