The search feature into mojoportal is based in the Lucene.Net engine, so you have code for indexing and code for manage the search, and the results.
The code for indexing is find under mojoportal\mojoPortal.Business.WebHelpers\Search\
The code for the search control can be found under mojoportal\Web\Controls\SearchInput.ascx
The code for the result page can be found under mojoportal\Web\SearchResults.aspx
You also have to handlers one for the search results under mojoportal\Web\SearchResults.ashx.cs and one for describing the search engine \Web\SearchEngineInfo.ashx.cs
And for your last question look into http://www.mojoportal.com/contribute.aspx