Try using the Edit Node cursor, however note that you've got 2 lines, exactly on top of each other so their selection points will be in exactly the same place. For the purposes of selection, the thickness of a line is ignored and you can only click on the 'hairline' element of a line to select it.

As the black line is on top of the yellow one, you'll wind up selecting it as it's at the top of the stacking order.  There isn't an easy way to select the bottom-most of two identically shaped, exactly over-lapping objects; Stardraw is designed to select the topmost object at the co-ordinate where the selecting click occurs.

Kind regards,
Rob Robinson
Stardraw.com

While I am at it, I don't see this question addressed.  I use the cable tool, copy the cable line add some width to it, change it to yellow and send it to the back to highlight new work, or upgrades.  This way the line already perfectly lines up with the line I am highlighting.  The problem comes when I need to change its path.  I can't seem to quickly grab the thick yellow line.... eventually I can, but it doesn't make sense and isn't easily repeatable.  This is just an efficiency question.  What is the trick?