FFT Megawizard problem with I/O data flow of Variable Streaming

This is the first time that I use the IP of FFT. When choosing the I/O data flow of Variable Streaming architecture, I set the sink_real as a sequence of sine and let the sink_imag be 0.

Having Simulated in Modelsim SE and Matlab and checked the wave,I find:

1: there is source_real data output but the source_imag is 0 in Modesim

2:simulated <module_name> model.m in Matlab, the result of <module_name>model function is almost equal to the result of fft function of Matlab with a little error.

These 2 questions troubled me for two days . How can I make it?