It seems that you put the source/header file at the Workspace folder like following.
Because the project does not include this folder as the search path, PSoC Creator cannot find these files.
Solution 1: Move the files to the project folder
After moving these files, PSoC Creator will miss these files.
Please delete these files at the "Workspace Explorer" and Add these files with a new location.
Solution 2: Add a folder to the include path.
Additional search paths can be set. Please select the Menu bar Project>Build Settings... to open following Build Settings dialogue.
There is a field named "Additional Include Directories" to specify additional paths. Just type two periods in the text field and click OK button. PSoC Creator can find these files.
I took a slightly different approach,
I removed bcc425UART.c and bcc425UART.h from the project,
then moved these to files to the folder where main.c resides.
And added them as "Add > Existing Item... "
Then when I tried to build the project, I got following errors
So I modified CY_ISR(UARTRxInt) as below
CY_ISR(UARTRxInt) // rotira de interrupcao
// copia os dados do buffer interno da UART para o buffer RxBuffer
// while(UART_ReadRxStatus() & UART_RX_STS_FIFO_NOTEMPTY)
RxBuffer[RxWriteIndex] = UART_UartGetByte(); // retorna o proximo byte do buffer interno da porta serial
// faz o buffer ser circular
RxWriteIndex = 0;
And the project is now compile-able.
Note: As I don't have the board, I have not tested if it works or not, though.