Uninstalling Mailenable then installing Smartermail is one way, you will probably want to check in the components management to make sure that it reconized that you have installed Smartermail. If it doesn't show up the first time make sure that your Smartmail services are running and then use the refresh link within the components management. Once you see Smartermail listed click the SmarterMail link and enter the information based on how you setup your Smartmail server.
Or you can install smartermail then go into Server > Components Management > Mailserver (this should be a link because Plesk will see two mailservers and it will give you an option of changing it out) Once you have selected the Smartermail server you will see fields for Admin port, Username and password, enter this data based on how you configured your Smartermail server.
If you run into problems or have questions PM me I can help you through it as I've setup several of our servers using Smartermail