Click on the cell in question, then click on Format, Conditional Formatting...
Under Condition 1, make sure it says, "Cell Value Is", and in the next slot, select "less than" from the pull-down menu; in the last slot, enter 0.1 Now, click on Format, select the red color from the Color: pull-down menu, and, if you wish, Bold. Then, click on OK, and again on OK. That does it!