There is no Lojban Sign Language. This page describes what would be involved in creating one. There are two major approaches to creating a Lojban Sign Language, each with different strengths and weaknesses.
Signed Exact Lojban
Signed Exact Lojban is not strictly a sign language, but rather a manually coded language: a representation of spoken Lojban in a gestural-visual form. The grammar of Signed Exact Lojban matches the grammar of Lojban. In this way, the sign language is is really an alternate morphology for Lojban.
In the simplest form, there would be one sign for each Lojban letter. A more complex Signed Exact Lojban would use unique signs for letter clusters.
Signed Exact Lojban would likely be more cumbersome to use that Lojban Sign Language. It would have the advantage of serving as a memory aide by giving a learner two ways to learn identical morphological units.
Lojbanistan Sign Language
Lojbanistan sign language is a signed language designed with the same goals and principles as Lojban. The grammar is different--taking advantage of the ability to work in 3 dimensional space instead of a linear aural channel, as well as being able to express multiple things at once (facial expression, arm movement, and hand shape.)
Probably features include typological features such as topicalization and SOV, those being very common among sign languages.
In order to be logically unambiguous, Lojbanistan Sign Language may be structurally quite different from other sign languages. As with Lojban, it would be desirable to create unambiguous statements with relatively short expressions. One possibility is define space for individual predicates, such than an x1 place is always in the same location in a conversation.