The Bioconductor project changed a bit the internals of its package installer. The following should work:
from rpy2.robjects.packages import importr
# do the following _only the first time_, to install the package seqLogo
base = importr('base')
base.source("http://www.bioconductor.org/biocLite.R")
biocinstaller = importr("BiocInstaller")
biocinstaller.biocLite("seqLogo")
# load the installed package "seqLogo"
seqlogo = importr("seqLogo")
Otherwise the bioconductor extensions to rpy2 have not been updated for quite some time. There might be other things that would need to be fixed.