So, I locked an exit between the rooms "Cabin" and "wilderness". The prerequisites:

1. I need to speak with the man in the cabin before he lets me out the back door (there are two exits in the cabin).

2. I need to set a flag before I can speak to the man in question.

So, the flag was set, I arranged the necessary if-statements underneath the man's speak options, also checking the box "unlock an exit" between said rooms...but the exit is still locked. What is the problem here?

Can you attach your ASL file?

define room <Cabin>
prefix <the>
look <|s09The interior of the cabin is dreary. There are doors to the east and south. There's a man inside the room.|s00>
south <Front Yard>
east locked <Wilderness; You cannot go there yet.>
script picture <cabinX.jpg>

define object <Lucero>
speak {
if flag <miller kidnapped> then {
unlock <Cabin; Wilderness>
msg <|s00"What? Doctor Miller's been kidnapped? Alright, Baron Labs is up ahead. Be careful. My name is Lucero. Should anything happen, contact me on this radio. I'll help you out.|n|nBy the way, you can have this radio. It might come in handy. And beware of the Dezolis Cat to the north!">
if not got <radio> then {
show <radio>
give <radio>
msg <Lucero handed you a radio.> } } else msg <He seems friendly enough. >
}
displaytype <Character>
article <him>
gender <he>
end define

Your "unlock" command looks incorrect:

unlock <Cabin; Wilderness>

should be:

unlock <Cabin; east>

I realise this isn't actually very clear from the documentation, but you use a room name as the second parameter for "go to (room name)" type exits - for directional exits you need to use the direction name instead.