I have a search form with some fields from the view I use to search in. In NH I would use QBE is there something like that in LLBLGen.

Here you can use IPredicateExpression and IRelationPredicateBucket and add in those almost anything. You also can use natural language to add criteria:

IRelationPredicateBucket filter = new RelationPredicateBucket();
// this is a LIKE predicate
filter.Add(CustomerFields.CompanyName % "%some company.."%);

So, if you have a search form, you could something like:

if (!string.IsNullOrEmpty(txtFirstName.Text))
{
     filter.Add(CustomerFields.FisrtName == txtFirstName.Text);
}

if (!string.IsNullOrEmpty(txtLastName.Text))
{
     filter.Add(CustomerFields.LastName == txtLastName.Text);
}

...

// then use that IRelationPredicateBucket at your fetch routing

If you are using LLBLGenProDataSource(2) to bind the results to some control (say a grid), you can filter on-the-fly without write any code.