syaknikwa - Your monitors only have XLR or 1/4" jack inputs. So you cannot use regular RCA phono cables at that end.
It doesn't matter though. Since balanced cabling only works when both ends of the connection are balanced and your interface doesn't do balanced out there is no point in using 3 wire XLR or TRS plugs. Regular hi-fi two wire plugs will almost certainly be fine in your circumstances.
The cheapest solution if you already have a suitable length of RCA phono to RCA phono cable lying around is to get a pair of these RCA phono to 1/4 TS jack adaptors.
Put one into each of the jack sockets on the back of the monitors, split the RCA phono cable into two strands and connect your monitors as normal..
If you want to get new cables then type the desired spec (starting at the source end) into google and you will find several local suppliers easily.
In your case you are looking for "cable rca phono to 1/4" TS jack".
This is what I found 1st on list.
http://www.amazon.co.uk/Phono-RCA-inch-Jack-Leads/dp/B00555JSWQ
Then, assuming the volume pot on the NuForce controls the volume of the line outs as PM suggests, you are now good to go. All for less then $10.
Balanced TRS or XLR cable does have certain advantages but I don't think any of them are relevant to your situation.
Balanced is usually 6dB louder than unbalanced - this doesn't matter to you because you are unlikely to be short of gain and anyway the 305s offer a Selectable Input Sensitivity switch on the back. So you can use the max power with any input if you follow the setting instructions in the manual or website.
Balanced plugs are more robust and reliable - this doesn't matter to you because you are unlikely be setting up and breaking down the system very often.
Balanced cable is more resistent to interference - this is unlikely to matter to you as you are using short lengths in a domestic setting.
Balanced cable makes it easy to lift the ground to avoid earth loops - this probably will not matter for you as any half decent interface or DAC should be galvanically isolated as a matter of course.
