Applying a function to a Dataset column conditional on another slot's value

Here is one way:

data[All, MapAt[foo, #, #PLATFORM /. {"Dropbox" -> "LOCATION", _ -> {}}] &]

Or, if one prefers If:

data[All, MapAt[foo, #, If[#PLATFORM == "Dropbox", "LOCATION", {}]] &]

Edit

The preceding expressions use MapAt with a fixed function (foo) and a conditional set of parts (either "LOCATION" or an empty list). As @alancalvitti notes in the comments, MapAt can also be used with a conditional function (either foo or Identity) against a fixed part ("LOCATION"):

data[All, MapAt[If[#PLATFORM == "Dropbox", foo, Identity], #, "LOCATION"] &]

This is essentially the same as @LeonidShifrin's solution.

Edit 2 - Dynamic Query

We could also apply a dynamic query operator:

data[All, Query[If[#PLATFORM == "Dropbox", {"LOCATION" -> foo}, All]]@# &]

Perhaps, there is a better one, but here is one that comes to mind:

ClearAll[transform];
transform["Dropbox"] := foo;
transform[_] := Identity;

and then

data[All, With[{tr = transform[#PLATFORM]}, MapAt[tr, #, {Key["LOCATION"]}]] &]

Okay, this question is already answered, but I want to add my 'preferred way' here:

First, we define a new operator:

ApplyIf[f_, g_, x_] := If[TrueQ[f[x]], g[x], x];
ApplyIf[f_, g_][x_] := ApplyIf[f, g, x];

Then we do the query:

data[All, ApplyIf[#PLATFORM == "Dropbox"&, MapAt[foo, "LOCATION"]]]

I think it's pretty clear -- we're using composition to nicely separate the condition from the action.

ApplyIf should probably exist in the core language. The particular compound Map[ApplyIf[test, f]] is extremely common, I find.