I know that a bar over your variable isn't something this applies to, but if you solve that problem and need other math symbols, try using the character palette.
Go to System Preferences and the International pane. Under the Input Menu section, put a check on Character Palette and a check on Show input menu in menu bar. You should also scroll down and put a check on your main keyboard layout (which is probably U.S.).
This will put a little flag in your menu bar; when you click on it, you can select Show Character Palette.
When that comes up, change View to All Characters. Find the Symbols section on the left, then Mathematical Symbols under that. This will list ALL the math symbols Unicode knows about. The window will show you all the fonts you have that have that symbol in them. Double-clicking on one will insert it wherever you're typing.
If you install that and then open it pointing to, say, Times Roman, you'll see that one of the categories is "Combining Diacritical Marks"; these are characters that, if typed following a character, are combined with that character.
One of the combining diacritical marks is "Combining Overline"; it will put the overline over the preceding character.
NeoOffice and OpenOffice have an equation editor like Word's, and they are free. As is Latex, but the interface is point-and-click. It looks like you can't generate the symbols and then copy them out to TextEdit, though.
In the Character Palette
> By Category > European Scripts > Diacritics
find and selected "combining overline" [the "bar"]
in your application type the letter you want the bar above
then click insert: x̅
My initial kludge was to search for "x bar" in google, and find a hit where the character appears, copy it to the clipboard, and it is yours.
Things are a little different in Yosemite.
You can set the xbar symbol as auto-replace text in aware apps (such as Safari, Notes, TextEdit - but NOT Microsoft Office: see below).
To get the xbar symbol you can create a Unicode diacritical compound character (which overlays one character on top of another). The one you want is "overline" or "macron", which are Unicode Hex option0305 or option0304. To use Unicode Hex you need to have/add the Hex keyboard (System Prefs > Keyboard > Input Sources > add "Others" > Unicode Hex Input). This places a flag on the keyboard (e.g. US flag if your default keyboard is U.S.) and adds a Keyboard menu with the Hex keyboard as an alternative option (with symbol U+).
With the Hex keyboard selected, in a word processor, type the letter "x", and then holding the Option key, type 0305 (or option0304). If your word processor is capable of showing Unicode diacritical characters, you should now see the xbar symbol x̅.
Now for the clever bit: copy the compound character, and in Keyboard Preferences (now showing in the Keyboard menu) choose the Text panel, and add in the Replace column the letters "xbar". In the With column, paste your compound xbar character. Any time you type the letters "xbar" followed by a space or a pause (in a compliant App), MacOS will suggest the new character. You can reverse the substitution by clicking the cancel box, Escape, or Undo.
As Microsoft Office is not compliant, there is a second method for Word etc. In Word, choose Tools > AutoCorrect. There are two options here: in the AutoCorrect panel, you can type "xbar" in the Replace box, and paste the compound character in the With box. Alternatively use the Math AutoCorrect panel, and use "\xbar" in the Replace box to constrain its use just to "math regions" (Equation boxes). You could also check "Use Math AutoCorrect rules outside math regions" to use any of the preset commands within Word, but this substitutes many characters - so use cautiously. The substitution kicks in after a space, and can be undone with Undo.
- Press Ctrl+Command+Space to open the Viewer (or open using other means in above link)
- Type "combining overline" (search bar is revealed when you start typing)
- Click on the symbol to insert it (modifies preceding character)