As DRT indicated, one option for you would be to enter an order to SELL a certain USD amount of "JPY/USD Cash USD.JPY" (Underlying = USD), quoted at 117.xx. That's the cash forex dollar/yen pair at IB.
Option #2 would be to enter an order to BUY 1 or more of "JUN06 Futures 6JM6" (Underlying = JPY), currently quoted at 0.0086yy. That's the
Globex Yen future contract. (There's also one on EUREX.)
Option #3 would be to express your long-term dollar / yen view via options. Sounds like you want to keep it simple, though, so let's leave that one off the table for now...
Normally, for a long-term play, you'd want to deal in cash, not futures -- everything else being equal, which it is here, with the same broker. First, no worrying about rollover 4 times a year. Second, more flexible trade sizing and ability to scale in or scale out.
However, there's more to it than that. As usual, devil's in the details... Before you take the plunge, are you fully aware of the interest costs? Short USD / long JPY is a reverse carry trade. It'll cost you dearly at IB. Right now, you'll pay 6.295% on the USD part (up to $100,000) and you'll also pay (rather than get paid) from 0.439% to 0.189% on the JPY part, for a total of 6.5%+ annual drag. See
this for details.
So, let's say, your directional forecast turns out to be correct. If you were to hold this position for a year, and yen ends up at, say, 106 (a nice 10% appreciation against the dollar), your total return would be only about 3.5%. That's ignoring any further widening of the interest rate differential (IRD), yet to come this year. And before taxes, if any.
To be fair, that % return is applied to the (leveraged) position size, not your account balance. E.g., with 5:1 leverage, you'd make around 17.5% total return in that (good case) scenario, before any taxes.
On the other hand, if you were to put on this position via futures, not cash, the implied IRD would still work against you, of course, but at a more reasonable 4.5% or so, currently. That's an extra 2%+ a year unleveraged, or extra 10%+ P&L at 5:1, and so on. If that matters to you, you need to weigh that extra futures P&L against the cash advantages for this purpose, mentioned above. Your intended 1) position size, 2) leverage and 3) time frame would be the key factors in a decision of cash vs. futures as the best tool for the job. Feel free to post them here, for more specific advice. Have fun.