I am very tired and drunk and am just writing this without trying it but here it goes.

flag var bit
flag = 0

start:
If portb.0 = 1 and flag=0 then
portb.1 = 1
flag = 1
pause 1000
end if

if portb.0 = 1 and flag=1 then
portb.1=0
flag = 0
pause 1000
endif
goto start

The flag is a way for the micro to keep track of which state is currently on. The pause allows the user to remove there finger from the button so the LED is constantly flashing. This may neet to be adjusted. This will be 1000 ms.