I guess You have a general problem.
The RS485 Chips are only to make the signal You want to sent compatible with the used Cable.
The serial signal had to be produced and received by the Microcontroller.
There are to set some registers of the USART to configure the bit rate, parity handling an the number of data bits.
(
Take a look into the Datasheet Chapter USART Page 94).
After setting this, You can have to look into a specified register if the data buffer is empty.
If it is You can write a byte into the Data Register of the USART and it will be transmitted automaticly.
At 2 wire Connection the RS485 Chip has to be configured as transmitter by a additional Port befor sending.
Receiving works similar.
When a data byte was received a bit was set in an receive status register of USART and You can readout the USART Data Register then.
The better way is to use USART receiver interrupts and copy the received Bytes into buffer array.
But that's a little bit more complicated.
Basicly that works with any program language similar.
In some IDE's a function is preprogrammed to set the depending USART registers.
You only had to call that routine with the wished parameters and the USART Register will be wrote with the right values.
What IDE You wanna use?
I wanna suggest You to change to a little bit more modern controller like the ATMEGA 16.
For first experiments You can use the CodeVision AVR IDE then, that has a Automatic Programm Generator to avoid exactly Your register problem.
Be aware, the free version is Code resticted at 4k Sourcecode.
For later use You can use ATMEL ( now Microchip ) STUDIO 7.
I use CodeVision to configure the used Hardware and Copy the generated Code into Studio 7.
A few names and defines had to be changed, but in sum that works good and go fast.