You could start with the example file attached (it can also be found in your OptiSystem Samples folder) and work from there. In [1], they get 1000 km of propagation with a BER of 10-3, although they don’t really describe their amplifier setup. Also their multiplexing format is not exactly OFDM.
[1] W. Shieh, Q. Yang, and Y. Ma, “107 Gb/s coherent optical OFDM transmission over 1000-km SSMF fiber using orthogonal band multiplexing,” Opt. Express 16, 6378-6386 (2008).