How can I use \if \then \else with \@ifclassloaded{}

You can use etoolbox and its \ifboolexpr command to combine several conditionals of the form \<conditional [possibly with additional arguments]>{<true>}{<false>} into one.

Note the test keyword in front of each conditional and the braces around them.

\documentclass{article}

\usepackage{etoolbox}

\newcommand{\test}{} % just to make sure \test is undefined

\makeatletter
\ifboolexpr{   test {\@ifclassloaded{book}}
            or test {\@ifclassloaded{report}}
            or test {\@ifclassloaded{memoir}}}
  {\def\test{lorem}}
  {\def\test{ipsum}}
\makeatother


\begin{document}
\test
\end{document}

---

In this example you could also directly test for the existence of the \chapter command. Depending on what you are trying to do that may actually be the better idea.

Here I used \ifundef from etoolbox, this reverses the logic from the first example. (There is also \ifdef, but that treats commands that are \relax as defined, which isn't that useful for this application.)

\documentclass{article}

\usepackage{etoolbox}

\newcommand{\test}{} % just to make sure \test is undefined

\ifundef\chapter
  {\def\test{ipsum}}
  {\def\test{lorem}}


\begin{document}
\test
\end{document}

The syntax of \@ifclassloaded is

\@ifclassloaded{class}{yes code}{no code}

so you do not need \else. Both branches are built in to the syntax.

You have

\@ifclassloaded{book}
{%
<code block>
}
\makeatother%

so your "no code" argument is \makeatother so you only execute that if the class is not book.

You should never have \makeatletter or \makeatother in a package code anyway so your fragment should look like

\RequirePackage{pgffor}

\@ifclassloaded{book}
{%
<code block>
}{%
 else code
}

\@ifclassloaded{article}
{%
<code block>
}{%
    else code
}

Incidentally checking for class by name isn't really recommended, there are thousands of classes and if someone makes one by copying book.cls and changing a few things, your package will not recognise it as a book-like class. It is usually better to test for specific features like \chapter being defined rather than checking that the class is called book.