The message "No product SKU found" is a very common one and the cause is almost always the same. Delimiters.
When importing a file, usually a CSV file in this case, the data is separated by delimiters. For CSV Improved to understand the file that is being imported it needs to be told what delimiters the file is using. Let me show you with an example.
The delimiter setting in CSV Improved:
The file:
~product_sku~^~product_name~^~category_path~
~Wb1~^~Product1~^~Home/New Products~
~Wb2~^~Product2~^~Home/Products~
~Wb3~^~Product3~^~Home/Products~
The ^ separates the fields and the ~ encloses the text. Now CSV Improved knows how to read the file. Make sure both delimiters always match.