quotes in mathmode?

[Bill Simpson]

I want to do, in mathmode,
Pr(``bigger'')
I tried
\Pr(\mbox{"bigger"}) using double quotes.
The quotes are absent when I veiw with dvi.

I tried
\Pr(\mbox{``bigger''})
but I can't get Lyx to "accept" the single left-hand `` quotes.
When I type them they don't show up at all. (Also absent from dvi
view--the right hand ones '' show up fine)

Please tell me what to do. Thanks!

I am using Lyx 1.0.0pre2

Bill