Follow the step below, maybe you can get what you want. 1. Click on the button you refer to, the one that seems to have a plus sign shape in the upper left corner, and a squiggly line in the lower right corner - if you hover your mouse over the button, you'll see it's called the Freehand tool. To make a straight line with the freehand tool, click the tool, then in your document left click to create one end of the line, RELEASE the mouse button, then move the mouse to wherever you want the line to end, then left click again.
2. If you click the triangle in the lower right hand corner of the freehand tool, you'll get a flyout menu of other tools, the second of which is the "two point line" tool. To use that to create a straight line, click the tool, then in your document left click where you want the line to begin, HOLD the mouse button down and drag to wherever you want the line to end, then release.