I may have inadvertently been confusing, sorry. If I have extra cash I could, in order of return, keep it at my broker in some kind of sweep account, buy a treasury, or buy a box. You can buy the box at a discount to it's guaranteed value at expiration, that difference is the interest just like buying a treasury at discount to face value. That's the buying a box to earn interest part.
Alternately, you may want to borrow money using a box. In that case you sell the box, at which point you can use the proceeds to buy stock effectively getting the same 2X leverage as you would if you used your broker's margin. In this case you sell the box for less than you end up paying when it expires, that difference represents the interest you paid to borrow. It's crucial that you sell the box, then buy the securities you want to effectively margin, not the other way around as it won't work unless you have the cash from the box sale in your account first.
Because you can generally trade just off the mid, the guy buying and the guy selling are getting/paying pretty close to the same effective interest rate. Unlike your broker, for example, where the spread on the interest you get from your idle cash and the interest you pay for a margin loan is generally substantially bigger.
Why the example on IB didn't work I have no idea. I did once do this with IB when I had an account with them. I don't have an account with them any more because they're truly awful in a number of respects, not least among them their inability to correctly calculate the max possible loss in an option spread and their penchant for auto-liquidating based on that incorrect calculation, so I can neither defend their platform for this purpose nor recommend you use them for this (or anything else besides straight, unlevered stock trades for that matter).
Hopefully that was a little clearer? Definitely worth running through the mechanics in a demo account a couple times to play with it.
