If an IATI data file fails to validate against the schema, this is not the end of the world!
Files fail validation for many good, practical reasons from a publishers point of view, and yet still provide useful accessible data for other people to use.
However, valid files do make it much easier for data users to work with your data, and passing the 2 basic rules of:
should be seen as basic requirements of your data
If you want to create a simple link for other people to come to the validator and see results on your IATI XML, simply add a 'url' query parameter to the address.
http://validator.iatistandard.org/?url=<your URL here>
xmllint is a really useful tool for checking file validation on your local machine. Information on how to use xmllint with IATI data can be found on our wiki.
If you want to place the schema files on your local machine then be sure to place ALL the schema files in the same directory as they each reference each other.
You can find the schema on iatistandard.org or in the IATI-Schemas GitHub repository.
The W3C validator, that many developers are familiar with for checking their web documents, can be used to check if a file is well-formed, but you cannot use it to validate against the IATI schema. As such you should take some of the warnings with a pinch of salt. Essentially the validator is a DTD based validator and IATI uses XML Schemas instead. See XML DTDs Vs XML Schema
There are some online services that allow you to upload XML and a Schema against which to test. These FAIL on IATI documents because IATI uses linked schemas that reference each other. So in effect you need to upload 3 schema to be able to check your document.
Published IATI data can be found on the Registry at http://iatiregistry.org.
The IATI knowledge base and support system can be found at http://support.iatistandard.org.