I'm not a TeX expert, so it's not 100% clear to me what you're trying to sum. I would think that to "calculate a value" you would need explicit limits for the indices. I also suspect you may have errors: it looks like you didn't subscript parts of your expression.

I would suggest two approaches: (1) Look at Mathematica's documentation for ToExpression. You can specify TeXForm in ToExpression, and once you clean up your TeX, that might work for you. (2) Try to use Mathematica syntax and re-create your equation from scratch in Mathematica. This will involve defining custom functions as well as using Mathematica built-in functions like Sum.

Maybe if you could explain what your expression is trying to compute or what you are trying to achieve, I could be of more help.