Chromatic Aberration can be a bit tricky, and in many cases you can't actually correct the fringing, only the color cast, caused by CA. In LR 3, you have two ways to correct lens aberrations. The first, and most simple, is to use a lens profile which should automatically correct for ALL lens aberrations in your shot, including CA, distortion, and vignetting. The other option is to manually correct for those aberrations.
To manually correct for aberrations, go do Develop mode and expand the Lens Corrections panel. Click "Manual", and scroll down to the Chromatic Aberration section. You have three controls here, the
Blue/Yellow sliders, as well as a
Defringe options list. When using the sliders, the general idea is to shift the slider toward the opposite color of CA that you are seeing in your photos. In many cases, you will only have one axis of CA, however in the worst case you may have both axes. When both axes of CA are present in an image, you might see other colors, such as green, which require an adjustment of both sliders to fully correct.
To make life easier when correcting CA, try this little trick. Hold down the ALT key and adjust one of the sliders. This will limit the photo to just the two color channels affected by that slider, making it a lot easier to see the effect of your corrective adjustment.
The Defringe options allow you to limit where LR applied "defringing". CA is an optical effect that results from the divergence in the way different wavelengths of light focus. Particularly around the edges of objects, this can cause a slight halo or blurring, creating a soft fringe around the edges of those objects. Defringing attempts to correct this halo. Sometimes correction of CA itself will be sufficient and leave no halo, however if one is left behind, you can try the
Highlight Edges or
All Edges options. All Edges may leave unslightly "hard" edges, or double edges with a thin hairline of dark between the object and another edge. If that occurs, try Highlight Edges.
Correction via Profile
In addition to manual correction of optical aberrations, you can also use a lens profile to correct for all aberrations at any focal length. LR 3 comes with numerous lens profiles out of the box, and it is also possible to create your own. You have some ability to adjust the three types of lens aberrations corrected by a lens profile, but not as much control as with manual.
If your lens is not included in the list of profiles out of the box, you can use the Adobe Lens Profile Creator to create your own. The process can be a little tedious, but it can be handy when you have a LOT of images to correct on a regular basis.