Assuming you already know about how to create an catalog (refer this post) and you want to learn How to open existing catalog refer this post. Follow below steps to open your existing catalog.
Following are some errors and their resolution while opening an Data Manager Catalog.
UDA-SQL-0532 : Data Source is not accessible: "<ServerName>". Check Server Name
UDA-SQL-0031 : Unable to access the "ALIAS_07FA1C10" database. Check Data Base Name.
UDA-SQL-0564 : [Microsoft SQL Server Native Client 10.0]Login failed for user '<DomainName>\<UserName>'. (SQLSTATE=28000, SQLERRORCODE=18456). Check your login information. Is Current user have proper access.
UDA-SQL-0564 : [Microsoft SQL Server Native Client 10.0]Cannot open database "<DatabaseName>" requested by the login. The login failed. (SQLSTATE=42000, SQLERRORCODE=4060).Check Database Name. Is Database exists. Is user have proper access to use database.
Please login to post your valuable comments.