On the folder bar, click on "Insert", then it show the insert folder. On the far right should be something called "Symbol". Click on it and box will appear a drop down box with some recent symbols and "more options". If those choices aren't what you're looking for click on the "More Options..." then you should find mu and sigma and square root but i have i don't think they have xbar though.
Update: Sigma upper case ∑ is on the 2nd last row of the table, 2nd last column to the right; Sigma lower case σ is on the 2nd column from the left, 12 rows up (click on the scroll bar to scroll up) from the bottom; square root √ is at the bottom row, 2nd column.
I copied and pasted these from MS Word so you should be able to find them.
Update 2: I found an xbar from the Internet. Simply just point your mouse pointer on the x bar symbol and select it (click and hold the left mouse button and drag it to the right until the symbol is highlighted), copy (click on the right mouse button and select copy from the pop-up menu or click on ctrl-c) and paste (open your Word document and put your mouse pointer where you want to put the symbol, click on right mouse button and select paste from the pop-up menu or click on ctrl-v) it to your word document: x̄
If it's easier for you, you can just copy and paste symbols i already put here and it should let you.
Update 3: That symbol ∑ I mentioned earlier above is not upper case Sigma, that is an N-ary summation. It looks like upper case signma but it is larger.
Upper case Simga is Σ, found at 14 rows up from the bottom and 8 columns from the left.