[nikos kalp]"I have a client he wants me to export into this format : HQ 1920 x 1080 - 50i, 35 VBR , I don't see anywhere fcpx giving me any option with that specific format."
That request is a little confusing. MXF is a Media eXchange Format container and doesn't tell you what codec they want you to use. If by "HQ" they mean "ProRes 422 HQ" then that doesn't make sense because ProRes HQ is 220 Mbps.
The closest preset I see in Compressor is: 1080i25 XDCAM HD 422 50Mbps and you could easily customize that to deliver 35 Mb/s but that's using the XDCAM codec in an MXF container.
You should ask the client what codec they are expecting in the MXF file.