There are at least three ways:
1. Click on a line to highlight it and hit the Delete key on your keyboard.
2. Right-click on the line and choose "Delete" from the drop-down.
3. Use the "Power Trim" function under "Trim Entities" and click and drag across the line.
Please keep in mind that these work with Windows. I don't know anything about macs but I'd assume the functions will be similar. If not then at least this will get your question bumped up so maybe someone else will see it and be able to help.
Julie, it also depends on what line you're trying to erase. It may be part of a sketch which means you'll need to be in that sketch editing it to erase a line (which might lead to other troubles).
Show us a picture of that line and we might be able to better understand...