When romanizing Japanese you often need to represent the long vowel sounds in English with a mark above the letter, but because these "macrons" are uncommon, they can sometimes be difficult to produce. Below are methods for some of the different platforms.
Macintosh is set up a lot better than Windows for macrons. Simply open Language and Text Preferences, select the US Extended keyboard, and then you'll be able to make a macron by typing Alt+a and then the letter.
For the virtual keyboard on the iPad, simply hold your finger down on a vowel to bring up the macron option. This also works on Android tablets. For the extended keyboard, the iPad works just like the Macintosh. Go to General -> Keyboard -> International Keyboards -> English -> US Extended. Now you can type Alt+a and then the letter to produce a macron.
For years I thought typing macrons on Windows was impossible unless you were working inside Word, where Insert->Symbol gave you the ability to place letters with macrons into the text. I used to assign shortcut keys, and avoided working on computers that I had not set up ahead of time for this. There apparently used to be a way to insert macrons with the numeric keypad as well, but I either do not understand it, or it is no longer possible. Last year, one of the reasons I switched to Macintosh was to take advantage of the ability to use macrons in any application.
I am pleased to report that there are simple solutions out there with downloadable keyboards. The one I use comes from a Korean developer at Jōhō Tōgō Shinentai. Download JPMacron and you can insert macrons anywhere in Windows, just like on the Macintosh. The directions on the website are easy to follow for installation of the keyboard, and once you have it set up, you just press Alt + Shift to bring up the Japanese keyboard (as you normally would) and Ctrl + Shift to bring up the macron keyboard within it. Then, you type x before the letter you want to place a macron over. If you are writing in English, then you can just leave this keyboard on the whole time. That is all there is to it! The developer also has a keyboard for inserting tonal marks for Chinese Pinyin.