Hello Adekunle
There may be several issues involved in the successful communication between devices across an MPLS infrastructure. In order to get this to work, you must employ some sort of tunneling so that your IPv6 traffic is tunneled over the IPv4 MPLS infrastructure. One way to do this is described in this lesson that shows how to successfully achieve this using 6PE/6VPE:
Let us know some more details about your setup and at what point you are having difficulty so that we can help you further.
I hope this has been helpful!
Laz