How far off is your mouse position? 1-3 pixels don't sound like much.
Keep in mind that the mouse and OpenGL use different coordinate systems, depending on your model view, projection and viewport. You can use gluUnProject and gluProject to convert between the two.http://www.opengl.org/documentation/specs/man_pages/hardcopy/GL/html/glu/unproject.htmlhttp://www.opengl.org/documentation/specs/man_pages/hardcopy/GL/html/glu/project.html
Translate3f (x,y, 0) sets the origin of the object coordinate system to (x,y, 0). Depending on how your object (the disc) is drawn, that may be its center, its bottom left corner or an arbitrary point. It depends on where your object is located in its object coordinate system. In your case it's highly likely to be its center.