I am using JIRA 5.0.4 with which I have installed JIRA Agile 6.1.6.
Latest JIRA Agile supports Epic link and I am easily able to add Epic Link/Issues in Epic through UI but in older version I am not able to find Epic Link/Issues In Epic on UI.
I have already configured Epic Link to be shown on all the screens from screen configurations with Admin rights but still I am not able to find it.
From REST response I found that Epic Link is there with null value so there is Epic Link associate with entity but I am not able to add any value for Epic Link.
I have re-indexed JIRA after installing Agile plug-in. Also I have restarted JIRA server.
Update :
I have added it through API and it is visible now on UI but still I am not able to edit/modify it.