it looks like your port for the named instance is dynamic so without making any configuration changes you will have to get SQL Browser working.

First, is SQL Browser running? next can you telnet to the SQL Browser port (1434) on the Target server from the source server and vice versa, I am not sure which way the communication works with SQL browser. Also, do a telnet on the source server to port 3249 on the target server to check if that communication is open.

telnet targetserver 3249

If the network path is open the command prompt window will go blank with a blinking cursor.

if you don't want to try telnet you can try creating the linked server by using the name "servername,3249" although this may break when the target server is restarted because the named instance may choose a new port.

EXEC master.dbo.sp_addlinkedserver @server = N'SERVERNAME,3249', @srvproduct=N'SQL Server'