I have got this problem solved in my app.
Follow the below steps for CCAvenue.
1. Download plug in from below urlhttp://www.nopcommerce.com/p/264/ccavenue-payment-module.aspx
2. In the downloaded folder copy "Nop.Plugin.Payments.CCAvenue" folder to "Plugins" folder of your application
3. Open Description.txt file under the same folder and change supported version from 2.20 to 2.40
4. Now build your app and run
5. Login with admin creds and go to "Configuration > Plugins", here your will find "CCAvenue" under PaymentMethods group, click on install
6. After plugin installed click on configure and give ur merchant id there.
So CCAvenue plugin installed in your app and ready to use.
The final step is to make CCAvenue active under payment methods
7. Select "Configuration > Payment Methods" option from main menu and make the CCAvenue active by clicking on Edit.
I hope it helps for you.