What is the easiest way to transmit data from a computer to an FPGA?
Considering your request a classic serial interface will do the work perfectly. 

From a HDL perspective a serial transceiver is easy to implement, so I think you should start from here. Your board has no RS232 connectors, but you can easily use the expansion headers in order to connect a FT232 chip, that convert your serial interface to USB.

With a solution like this from both PC and FPGA you can use a serial interface and the FT232 will manage all the USB stuff, that is not so simple if you are just starting with HDL. In this way you have not to worry about USB and at the same time you are using a modern interface that every PC has, unlike the RS232.