Pre-built rockets/landers

[Arcamean]

I was looking around Spaceport and came across a few vehicles such as some small landers that would help me greatly in my science quest. I downloaded them fine, they show up but I can't select them for some reason, they're greyed out.

[RW-1]

Just my guess here ...

Are you playing in career mode? If so it's most likely because they are using parts you haven't unlocked yet, and should say so in the vehicle description.

After that, it's because the ship you downloaded uses non-stock parts, and you would have to get the mod that adds them to your installation to use.

[Clockwork13]

It's either what RW-1 said and you have unlocked parts and you are playing in career mode, or the craft requires parts that you don't have, in which case you have to download a mod to earn them.