The coast * ML term tests for HSP high/low dependent on coast. To test this fit the full model with method = ML and compare it to lme(HSP~coast+MBL, random= ~1|site, method ="ML") using anova(model1, model2). There are alot of technical issues with testing both fixed and random effects in mixed models, for details see past posts on the R-sig-ME list and also on the R Wiki (http://wiki.r-project.org/rwiki/doku.php?id=guides:lmer-tests). But lets ignore those, this should do OK.
The if coast*ML is significant then no need to go any further. If it isn't then repeat from the coast+MBL, deleting one of those fixed terms and repeating.