Usually, a thesis or dissertation acknowledgement is included in the very first part of your written work, situated in the first part of your written study. Generally, you should place it right in the middle between the title page, which opens your bachelor- or master- or Ph.D. thesis, and the beginning of your work. That can be for example the table of contents or in the case of a doctoral thesis the dissertation abstract. Some Universities provide guidelines of formatting. Check all specific requirements before submitting your thesis!