If you are using Oracle client software to connect to a database not on the current machine, you must set up an alias for your database. For example, you could use Oracle's "Net Configuration Assistant" or other such tool to set up an alias called "sun10" that has these settings:
You should then test this alias using SQL*Plus with a login of "username@sun10". Once this configuration is verified, you can set the ORA_LOGIN variable in the to that value, for example, "ORA_LOGIN username@sun10". If you prefer, this setting can be broken up using the A_ORA_DATABASE as follows:
ORA_LOGIN username ORA_PASSWD userpass A_ORA_DATABASE remotesrv
Information on Oracle's "Net Configuration Assistant" can be found in your Oracle documentation.