Hi
I have nearly the same problem with installing conn18b on Matlab 2018, though I was using conn toolbox with Matlab 2017 perfectly fine. 
Not sure what could be the problem. I reinstall Matlab and everything but it doesn't help.
Here is the error. I googled it but couldn't find anything helpful.


ERROR DESCRIPTION:

Undefined function 'conn_guibackground' for input arguments of type 'char'.
Error in conn>conn_menuframe (line 10252)
if isfield(CONN_gui,'background'), b0=conn_guibackground('get',get(ha,'position'),size(c)); end
Error in conn (line 103)
conn_menuframe;
CONN18.b
SPM12 + DEM FieldMap MEEGtools
Matlab v.2018a
project: CONNundetermined
spm @ /Applications/MATLAB_R2018a.app/toolbox/spm12
conn @ /Applications/MATLAB_R2018a.app/toolbox/conn