can someone enlight me.

if u use some thing like this..

/ip address find address=10.0.20.1

it doesn't give me any output... not even a 0 => interface number
is this normal? i read the manual. but i just can't find if it chould output something or if it can only be used in scripts...

this should not give you any output -- you have to do something with those addresses, f.e.:

/ip address find address=10.0.20.1

it doesn't give me any output...

try something like this:
:put [/ip address find address=10.0.20.1]
(don't forget the colon sign before "put")