If you are unable to login to Citrix XenApp Studio saying you do not have the permission to login citrix database may stored the wrong SID (changed due to user change etc.).
For this you'll need to know the administrators SID. You can find it in Active Directory with the attribute objectSID.
Do the following
1) Log into the SQL server where your database is stored with an account that has rights to your XenDesktop farm database.
2) Open SQL Server Management Studio
3) Click on Databases -> click on your database name (normally Citrix site name) -> click Tables
4) Scroll down to the DAS.Administrators table, right click, -> Edit Top 200 Rows
5) In the row edit window, replace the old SID with the new SID of the account you want to give admin rights to.
6) Close the edit window, and then reboot your XenApp/XenDesktop delivery controllers.
7) Sign into your delivery controller with the new account, fire up Studio, and you should be good to go.