Hi,
For this as per i think you should follow following steps:
1) As you already said you created a custom attribute in MV and storing E-mail ID into this attribute.
2) Now you have to create an attribute on FIM portal also and map it with this attribute.
3) After that Go to workflow and choose Function define and give name for activity display name like "Email".
4) On destination option click "lookup".
5) choose "WorkflowData" and in next option put "ActivityName" like i used "Email".
Now destination tab will look like "[//WorkflowData/Email/]"
6) Now, below this in value click concatinate value and choose your desired attribute from drop down and save it.
7) now add notification activity and in target option click lookup again and choose like step-5 or you can type directly "[//WorkflowData/Email/]"- must be same as step-5.
8) Define mail body and here you are good to go.
I hope it will help you to achieve your desired goal :) .
Thanks~
Giriraj