LyX ⇒ Cup with Dot

[eyenir]

I need to use disjoint union (and I can't use \sqcup). In the symbols guide I saw there is the commands \cupdot, \bigcupdot which appears to be in MnSymbol.

What I did is to install the package MnSymbol by the MikTeX Package Manager (as an admin). After that in LyX I went to Document -> Settings -> LaTeX Preamble and in the text box added \usepackage{MnSymbol}. Then I clicked OK but when I press on "add formula" the computer doesn't recognize cup with dot tags.

How can I solve it?