Ran out of salt can I use MSG (monosodium glutamate) instead?
Sure you can. It will not taste salty, but it also will not taste bland, it will have a "seasoned" taste. Will you like it that way, or miss the salt? That's not something we can predict (or you, for that matter, if you have not tried it before). You have to taste it so you can know if you personally enjoy eating saltless, umami food. 

When trying, make sure that you are using very small quantities. Measure in knifetips, not in teaspoons, add one at the time and stir well. You can easily overdo it, and there is no way to go back. Pay speciall attention to the feeling of "It's not good yet" - it can come from "not enough seasoning" or from "not enough salt", in the second case adding more MSG won't help.