To capture packets on a port, simply click Capture -> Start Capture as shown in the image below.
The capture mechanism of Xena L2-3 is based on leaky bucket model. When the incoming traffic rate is too high, the buffer on the test module card (FPGA) becomes full very quickly. As soon as the buffer is full, the capture stops automatically.
At low incoming traffic rate, ValkyrieManager can give you maximum 4096 consecutive packets per capture.
This 4096 maximum is by design.