What does it mean when the splash screen never shows up when I click on the mas90 icon?

This is generally a network problem.

Begin with checking the network using ifconfig and netstat.
ifconfig -a
net1: flags=4043<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.2.108 netmask ffffff00 broadcast 192.168.2.255
perf. params: recv size: 24576; send size: 24576; full-size frames: 1
ether 00:a0:c9:a5:0d:4d
lo0: flags=4049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 8232
inet 127.0.0.1 netmask ff000000
perf. params: recv size: 57344; send size: 57344; full-size frames: 1
atl0: flags=404a<BROADCAST,LOOPBACK,RUNNING,MULTICAST> mtu 8232
inet 0.0.0.0 netmask ff000000
perf. params: recv size: 4096; send size: 8192; full-size frames: 1

netstat -i
Name Mtu Network Address Ipkts Ierrs Opkts Oerrs Coll
net1 1500 mydomain bookkeep 2659732 127 4936817 204 732749
lo0 8232 loopback localhost 287327 0 287327 0 0
atl0* 8232 none none No Statistics Available
bash-2.03# netstat -rn
Routing tables
Destination Gateway Flags Refs Use Interface
default 192.168.2.1 UGS 0 0 net1
127.0.0.1 127.0.0.1 UH 3 287308 lo0
192.168.2 192.168.2.184 UC 1 0 net1
192.168.2.184 127.0.0.1 UGHS 1 7 lo0
224 192.168.2.184 UCS 0 0 net1
Check for errors in the /etc/hosts and /etc/resolv.conf files as well as entries on
the DNS.

bash-2.03# cat /etc/hosts
# @(#)hosts,v 6.1 1993/08/21 02:17:48 stevea Exp - STREAMware TCP/IP source
# SCCS IDENTIFICATION
127.0.0.1 localhost
192.168.2.1 proxy
192.168.2.184 bookkeep

bash-2.03# cat /etc/resolv.conf
nameserver 192.168.2.1
hostresorder local bind
search mydomain.com

It could be that there is a duplicate IP address on the network. Ensure that tcp is operating and that the port is listening.

bash-2.03# netstat -an |grep 10000
tcp 0 0 *.10000 *.* LISTEN

Verify that no other process is trying to use the port as well.
bash-2.03# ps -edf|grep 10000
root 659 642 0 Feb-01 ? 00:04:13 /usr/mas90/home/pvx *uxhost -ARG mas90cs 10000 A
root 642 1 0 Feb-01 ? 00:00:00 /usr/mas90/home/server.m90
mas90cs 10000 /usr/mas90 A