Got it! the 64QAM1 example you attached worked ok for me so I did not touch it. The other one was not working very well so I made some changes.
1) I changed the modulation frequency so that the entire bandwidth of the OFDM signal could be represented by positive frequencies, so around 5 Ghz.
2) I replaced the BER Visualizer with the BER Test Set, so I could use it to ignore some bits if I chose, for example cyclic prefix.
3) I also lowered the gain of the OFDM Demodulation because it seemed a little unrealistic.
4) The filter on the Quadrature demodulator had to be also changed a little.
The back to back transmission worked perfectly.