Is the word lernenswert correct?
Why is that? Is the word incorrect?
To start with your second question first: No, lernenswert is a well-formed German word, consisting of the root lernen ('to learn'), the linking element -s-, and the adjective-forming suffix -wert ('worth doing'). The translation given by dict.cc "worth learning" is therefore alright.
Why is lernenswert not listed in the Duden, then?
Well, first of all, as @c.p. pointed out, don't expect any dictionary to have all words of a language. Lexicographers always have to think about what to include. Words that are relatively frequent should go in a reasonably-sized dictionary, but you have to draw a line somewhere with infrequent words. With currently about 3,200 hits on Google, lernenswert is certainly not an extremely frequent word (compare, for example, the 500,000-odd hits for lesenswert or liebenswert).
Together with its relatively low frequency, 'lernenswert' is also a word that is formed using fully productive morphology. Basically, you can take the infinitive of any active verb and attach -wert to it in order to come up with an acceptable new word (it seems to me that the linking element is obligatory). For example, take schreiben, add the linking element, add the suffix -wert, end up with schreibenswert, which probably means 'worth being written'.
Note that this is somewhat different from a less transparent word such as 'liebenswert'. Morphologically, this word has the same structure as 'lernenswert', and its compositional meaning may be paraphrased as 'worth being loved'. However, this word has also an idiomatic, non-compositional meaning: if you speak about 'eine liebenswerte Person', you're more likely to be referring to a person that is acting kind and friendly, i.e. is showing kindness to others, rather than to a person who is worth being loved for whatever reason.
With words that are fully transparent like 'lernenswert', lexicographers have to decide whether they want to include them in their dictionary. Otherwise, they will quickly face an inflation of entries to consider: if you list lernenswert, schreibenswert, liebenswert, and lesenswert ('worth learning', 'worth being written', 'worth being loved', 'worth reading'), why not also list lernbar, schreibbar, liebbar, and lesbar ('learnable', 'writable', 'loveable', 'readable'), and why not also list Lernenswertigkeit, Schreibenswertigkeit, Liebenswertigkeit, and Lesenswertigkeit (roughly 'the characteristic of being worth learning', 'the characteristic of being worth being written', 'the characteristic of being worth being loved', and 'the characteristic of being worth reading')?
All these are grammatical words of German, but due to the regularity of most of them, lexicographers will most likely decide against including them.
So, to sum up, lernenswert is probably not in the Duden due to a combination of the following (and perhaps even more) reasons: