If you want to measure something on screen then you need to go into the measurement setting which is done by clicking RULER which you can find in the main tool bar. You can then click at the first point that you want to measure from, and a small Ruler window will then appear showing the length of that particular segment [DISTANCE = ] and also the total cumulative distance of the digitised lines or path [TOTAL = ]. Obviously at this stage with only one point on the map neither will give a reading.
The DISTANCE and TOTAL figures will now start continuously updating as you move the cursor around the screen. When you have the cursor over the end point of the line that you want to measure you can now press ESCAPE and this will freeze the values in the pop up box. Remember that you can use the zooming and panning tools to ensure that the points that you're choosing are really accurate.
That's it really. MapInfo makes measuring and dealing with straight lines on maps really very straightforward indeed.