How do you use it so it is gramatically correct?

Et al is correct.

Just use et al.

It would be better to write that sentence as

"This argument is supported in the text 'Sociology, fourth edition', by Van Krieken et al".

Always et al (it means and others so it can't be further pluralised)

I would say This argument is supported by Van Krieken et al in ‘Sociology, fourth edition’

Depending on your referencing you can just say Van Krieken et al (2000)

et al. is short for "et alia". it doesn't get pluralized. it is used simply as "et al."

But in your statement, the phrase is a possessive. If I were speaking, I would probably use "Krieken et al.'s text". In writing, I would change the word order to avoid the entire issue. (the argument is supported by the text "Sociology" (Van Krieken et al., 1994))

But if I had to do it in the way you want to, I would write it the way you have written it.

et al

It isn't really grammatically correct to use an abbreviation in the middle of a sentence like that. The full phrase is 'et alii' (or 'et aliae' if all the other authors are women). So you should say 'Van Krieken et alii's text'.

You could also use 'alia' instead of 'alii'. It's technically wrong (it refers to genderless objects, rather than people) but it's widely used.

et al - is enough