I have autocad 2007, I need to modify scale to 1:50, but I cant find, where. Help me please. 

Hello, please explain us what you want to find.
You want to modify your entire drawing --> No do your drawing at scale 1/1 and set the printing with a scale of 1/50.
You want to set your paper space port to 1/50 --> Do Zoom, enter, 1/50XP, enter to have your scale.
You don't have 1:50 in your scle list?
An other thing?

I want to drawing with scle 1:50, but I dont know where I should set it. In version 2011 was scle list in lower left corner, but in 2007 it isnt. thanks

The scale list should be in the tool bar "dimension".
You should be abble to customise your screen with different tool bars with a right clic in any tool bar in your screen, this should open a sreen were the activated tool bars are notched.

To my knowledge there was no way to set the scale of a viewport from the Dimension toolbar in AutoCAD 2007.  The three ways I remember doing it were 1) use the Zoom command with the Scale option (nXP), 2) picking the scale from a dropdown list that was found on the Viewports toolbar and 3) changing the scale within the Properties panel.In AutoCAD 2011 there is a way to change the viewport scale after clicking on the viewport frame via the Viewport Scale icon that immediately appears in the lower right-hand corner of one's screen.