First things first. You have to be careful when you install an oil cooler because they tend to cause a pressure drop in the oil system. If you use a crappy oil cooler, you get a higher pressure drop. First thing I would do is figure out what the pressure drop of the oil cooler you are trying to install is. Maybe call Mishimoto or check their website. If they can't give you the data, return it.
Second thing, unless this is a race car, make sure you have a thermostatically controlled valve to make sure oil flow doesn't go through the cooler until needed or you will actually hurt your engine long term. The oil will take forever to get up to operating temp and wont lubricate properly. Additionally, if you are doing all this work, you can use something like a Greddy remote oil filter setup that has the thermostatic valve in it. You get a thermostat and a remote oil filter which makes changing oil MUCH cleaner. I have mine mounted similar to the Maximum Motorsports oil relocation kit and it KICKS ASS having it there. No more wiping down my steering linkage every time I do an oil change.
Third.... an oil temp gauge is a great idea if you don't have one.
Fourth, adding this stuff makes the capacity of the oil system larger then stock. Make sure you add more oil then you did and check the dipstick when doing oil changes to make sure you get the right amount in there.
Now, on to your question. There are lots of ways you can mount the thing but I think making a bracket that comes off the bumper support or off of the radiator core support would be the way to go depending on the size/shape of the cooler and where the mounting holes are on the cooler itself.
