Well thank you for your help!!

I think I made this FullForm-mistake somehow also in the pre-fix version already, too. Without FullForm it works well in one line

Apply[List, Expand[Product[1 + C[i], {i, 1, 2}]]]

I would suggest that you not use things like postfix operators (eg, //) and so on until you have used a function in several ways first. Your example is not using Apply correctly. The direct way to do this (and avoiding using things like Out[-1] -- name your expressions and act on them by the name you;ve given them).

First,

result = Expand[Product[1 + C[i], {i, 1, 2}]]

then

Apply[List, result]

Do not use FullForm in defining your expression. FullForm is a way of printing out a result, but the head of the result is FullForm.

Head[FullForm[result]]

gives

FullForm

not Plus.

The only problem with what you tried is that you used the arguments backwards. Here is what you want to do for your case (and, sorry, I should have given you an example!):

Apply[List, f[a, b, c, d, e]]

then gives

{a, b, c, d, e}

Note that there is a very common shorthand for using Apply in this simple way. it's to use @@ in the following way:

List @@ f[a, b, c, d, e]

The function that you are looking for is Apply:

http://reference.wolfram.com/language/ref/Apply.html