LyX and Summation

The problem might be that you first have to type at least one of lower or upper limit, or need to place the cursor at the \sum symbol to be able to change the limit type.

1. "Insert > Math > Display Formula" (Crtl+Shift+M).
2. Type \sum, hit space.
3. Type _ as used for indices in math mode and type the lower limit (for example "i=1"), hit space.
4. Type ^ as used for exponents in math mode and type the upper limit (for example "10"), hit space.

There is some documentation, which is integrated in LyX. You can find basic information about math mode in "Help > Tutorial" in Chapter 4 "Using Math" and advanced information in "Help > Math" in Chapter 10 "Operators".

I found the answer. With the cursor just to the right of the summation symbol, click the "subscript" button to place the beginning of the limit below the sigma. With the cursor just to the right of the sigma, click the "superscript" button to place the end of the limit just above the sigma.

With the commands \overset and \underset, characters can be placed above or below an operator, respectively, to accent it.

source: http://elyxer.nongnu.org/lyx/Math.html [section 7.2]