Some of these datasets might be useful to you (depending on the language):
http://www.sfs.uni-tuebingen.de/GermaNet/
http://wortschatz.uni-leipzig.de/en/download/
https://www.spinningbytes.com/resources/germansentiment/
http://sentiwordnet.isti.cnr.it/
http://saifmohammad.com/WebPages/lexicons.html
https://sites.google.com/site/iggsahome/downloads
First you load the reference dataset into KNIME. Also, you process your original text with a Bag Of Words node and a Term To String node. Then you join your text with the sentiment data, giving each word in your text a sentiment if available in the reference dataset. Finally you aggregate the sentiments with a Group By node (e.g. mean), so that you get a total sentiment of your document.