The only issue I can think of is that the Management Server is on NT and the Database is on Linux, but I don't think that would be the problem.
If I go in to Enterprise Management Console standalone then I can add the missing database to the tree and perform actions against it, however it's backup and recovery that I really want to do so I need to use the management server. I can see 2 other databases that were created before the Management server!!
I am using Oracle Enterprise Manager version Oracle 126.96.36.199.0 and the database is Oracle9i Enterprise Edition Release 188.8.131.52.0.
I looked up your error on metalink and the problem may be whitespace or corruption in your TNSNAMES.ORA file.
I don't know how to paste the full link, however if you paste the following string into the search on metalink it will take you to the relevant post.
Failed to parse TNSNAMES.ORA file Error 100:NLNV String Format Error
If you don't have access to it then give me a shout and I'll post the whole reply to you.
There is lots of back and forth in the post but the final solution is:
. When the "EM Tools" parse the TNSNAMES.ORA file, it is finding either a white space in the very first position or something else has corrupted your files. Please recreate the TNSNAMES.ORA file either with Net8 Configuration Assistant or manually and make sure the first position of the file is not a space or blank line. You can also follow note: 164166.1 if you need more help.