Hi Soeren,
First you need to generate Setter and getter method
Go to get_method of the your filed value = 'not bound' remove the text not bound and put blank value like value = ' ' .
for making editable go to GET_I method of your field comment first line code rv_disable = 'TRUE'
( *rv_disabled = 'TRUE' ).
its easy way to but you need to write code for setting value otherwise you can follow what karthik as mentioned that is also correct
regards,
vishwanatha