DE2 board USB communication

Hello...

Now, i am doing my project to transfer image between host computer (PC) and FPGA DE2 Altera via USB and display it into monitor. I am still new in using NIOS II Processor. First, I try to buil project USB Device given in Altera CD-ROM (DE2_NIOS_DEVICE_LED.sof in DE2_Demonstrations folder). But, i found error when building project like below:

"The software setting (STF) file associated with this project is damaged. This may be fixed by copying your source files into new C/C++ application project. For more details see the error log."

What should i do? What is the meaning of copying source file into new C/C++ application project?

Thank you