There's 2 ways to do this (assuming that the plan you created is a Portfolio 2.0 plan):
The source of Portfolio plan is Project. Existing JIRA tickets dont show up on the portfolio to link. Nor does it allow to link tickets from Jira. My question is simple "How do you link an existing user story to an epic?" The funny thing is when i edit the plan i see the linked tickets under epic but when i confirm it, it doesnt show up under epic. Is there a simple way to do that?
I'm afraid you lost me there. I think everything is here for you to solve your problem. From what you are describing, my guess is that you are trying to link existing stories to epics from the list of epics in your Portfolio Plan. That is something you cannot do.
If you want to link existing issues (tasks, stories) to epics (which are at a hierarchically higher level in your plan), you should switch your scope view to story level as I described in my first answer. As I explained there, you just select the epic the story belongs to in the Epic column.
In other words, if you are trying to link items together in the hierarchy of your Portfolio 2.0 plan, you start at the lowest level and link upwards, not downwards.
Another big difference with Portfolio 1.0 is that you no longer have to import issues to get them in your plan. Whether you have defined a project, a filter, a board or more of those as a source to your plan, issues added in JIRA will automatically appear in your plan. That's why I also mentioned that linking your existing stories to epics in JIRA instead of Portfolio should also be a valid option to correctly link the two together and have the result automatically show up in your plan.
There is, however, a possibility that certain existing issues do not seem to appear in your plan, despite all this. That's why I also included a screenshot of possible reasons why this may not be the case.
So, with all this material, I would suggest to follow these steps:
Hello Walter, what i meant is once i have my Epic/User story in JIRA a ticket which is already created but it doesnt show up on Portfolio, how do i do that. In 1.0 i could simpy addup the tickets to the user story. But 2.0 doesnt allow me to do that, nor does it allow to easily add a user story to an epic. I have put a pic for your referenceNPI.jpg
What is the source of your Portfolio 2.0 plan? Is it a Project, a Board or a Filter? If your existing JIRA issues are part of the source of your Portfolio plan, there is nothing you should do specifically in order to get them in your plan apart maybe from refreshing your screen.
If you switch your scope to Epic view like this:
Screen Shot 2016-11-07 at 15.31.34.png
Do you see the indication of issues without parent epic at the bottom of your Epic list?
Screen Shot 2016-11-07 at 15.31.52.png
And while you are checking that ... are all your epics from your project visible in that same list?