You can do this a few ways. It looks like you are looking for a bandpass filter since a baseband filter by defination is centered at 0 and runns from -B to B as seen below:
Option 1 - Still centered around zero) If you are interested in the range from (+ or -) F1 (larger) -> F2 (smaller). You can work around this by using two filters. Just subtrack the F2 from F1. That should give you the band pass that you are looking for.
Option 2 - Still centered around zero) - Use a bandpass filter
Option 3 - Adjust the spectral center by using scale vi in the signal processing -> Signal Operations pallete
With any luck this should help you do what you want. If not please post back and let us know
I have acquired a band of 20 MHz IQ data at baseband (centered around 0 i.e the spectrum goes from -10MHz to +10MHz). This 20 MHz band contains 3 signals each of bandwidth 1MHz. Signal 1 is centered around frequency -6MHz, Signal 2 is centered around 0Hz and Signal 3 is centered around +4MHz. How can I filter Signal 1 i.e at -6MHz? since all the filters in DFD toolkit takes real signal as input and +ve cutoff frequencies.
What you need to do is break it into three signals and seperate and shift each one. After that you can can then you can use the baseband filter to do what you need.