There are 2 distinct left-margin areas in the Eclipse editors (at least some of the editors, including the Java editor), the overview ruler and the vertical ruler.
Click+drag line selecting has to be done in the vertical ruler area, not the overview ruler.
To select just one line, you can also triple-click anywhere on that line.
Update: If you have Code Folding enabled, those indicators are shown in the vertical ruler when you hover the pointer over an area that can be folded. Looks like this: