As Jimbojones says, there's no conditional formatting reason reason why it won't work
Try adding another number to that cell and see what you get. i.e in another cell put =cell in question + 5. Does it give an answer or an error.?
Also what formating are you trying to apply, if you just want the negative value to show as red text, you don't need conditional formatting, but you probably know that
I tried to give an easy way to test if it was a number, however I have realised that it doesn't work as it adds okay. Use the ISNUMBER()