The Curves adjustment command is one of the most powerful features of Photoshop, unfortunately it is not available in Elements. This is a summary of some of the ways to fill this gap.
There are a number of options with varying functionality, limitations and price.
Use Levels instead
The Levels command is available in all versions of Elements, however its functionality is very limited compared to the Curves command, but for many adjustments this may be all that is required.
Use Adjust Color Curves
As of Elements 7 there is an Adjust Color Curves command which is sort of a poor imitation of the Curves command. While this is limited, it is better than the Levels command alone.
http://help.adobe.com/en_US/PhotoshopElements/7.0_Win/WS3216BCB6-F54C-4c4a-99A0-EBEA69F30C27.html
Use Elements XXL
See Elements XXL for a great collection of over 230 extra features.
Use a Curves Adjustment Layer
In Elements 1 to 3 there was a hidden Curves Adjustment Layer that a number of people (myself included) provided tools to make this available. Unfortunately Adobe removed this as of Elements 4, however if you have an old version then this is still an option.
https://paulshipley.id.au/index.php/tools/extra-for-elements
Use Smart Curve
This free tool by Alois Zingl provides a fully functional curves feature, however it is Windows only.
http://free.pages.at/easyfilter/curves.html
http://www.earthboundlight.com/phototips/curves-photoshop-elements-4-5.html
Use Curvemeister
This is a commercial fully featured tool by Mike Russell which does everything that could be expected. It is fairly pricey compared to the cost of Elements, but there is a cheaper Elements only version and given what it does is still well worth the cost – especially when the alternative would be to buy the full version of Photoshop.
http://www.curvemeister.com/index.html
http://www.earthboundlight.com/phototips/curvemeister-2.html