When I go to 'system settings > add and remove software' and search for (for example) 'seamonkey', I get a list of seamonkey-related software. But how do I actually install it? When I click on Composer the ''apply' button remains grey and I don't see any other way of installing it.

At the right there's a gray-blue arrow which points down. Click it, it will become blue and then press "Apply".

Ah, that's a grey arrow in my case, which remains grey after clicking it. But the 'apply' button now becomes active and I can install the suite. Thaks for the help. However, this is not very intuitive and I don't see the point of having to click that arrow first. Is there one?
Btw, I first clicked the 'action' arrow at the top right, after which text from the different entries in the list started to overlap, making it partly illegible. I suppose that's a bug. But what is that arrow for anyway?