[TYPO3-templavoila] accessing selector box field

[Stephan Petzl]

i have tried to access a selector box field from within another field 
but there is nothing rendered (output is "xxxyyy")


             15 = TEXT
             15.value = xxx
             20 = TEXT
             20.field = field_header_visible
             30 = TEXT
             30.value = yyy

it seams that i cant access other fields too!
any ideas?