Question: Why some Dialog thesaurus terms cannot be selected?

Answer: As illustrated below, not all thesaurus terms can be selected for searching (this is true for all databases with a thesaurus).

Using MesH thesaurus (from Medline) as an example:

The terms without check boxes are those that the database producers acknowledge are used by searchers. However, they are not the terms that the database producer prefer you, the searcher, to use. They have a list of controlled vocabulary terms that their indexers will use to describe a particular document.

But, although these more common terms are not accepted search terms by the individual database producers, they acknowledge that they exist and thus will point you, the searcher, to the preferred term(s) to use to retrieve the relevant documents.

To achieve this, select a term, and it will automatically direct you to the preferred term. For example,  select “Brain Anoxia” from the above list:

The thesaurus will automatically direct you to the preferred term. Which in this instance is:
        Hypoxia, Brain

It will also illustrate why you have that new term with “use for” followed by your original term of choice: eg:
(use for - Brain Anoxia)