The new model B+ has a 40 pin rather than a 24 pin GPIO header.
- You can buy a 40 pin stacking header. Install Pi Power over the first 24 pins. This is the best option if you can find a 40 pin stacking header.
- You can obtain a 14 pin stacking header and stick that onto the bottom 14 pins of the GPIO header, raising them up to the same level as the other 26. You can also do this with two 7 pin SIP headers.
- If you're trying to connect a 26 pin IDC cable to your model B+ and don't need the bottom 14 pins, then just use your Pi Power as an adapter for your 26 pin cable. You won't need to snip off pins 27 and 28 like you would if you wanted to squash the IDC cable directly onto the 40 pin GPIO header.
Again, I haven't actually had a chance to try it yet, but so far as I know, Pi Power's 5 volt output should work just fine powering a B+ model.