Firstly - make sure that you have the SNMP set up correctly on both switches and Dude. Confirm this by double-clicking on the switch object in Dude and open the SNMP tab, then look on the Interfaces sub-tab and see that all the interfaces on your switch are listed.
Then, right-click on a blank part of your map and select "Add Link". Click on one switch and drag to the other switch. When you release the mouse button the Link dialog box pops up.
In that dialog:
select the device that will be the one that the link is bound to (one switch or the other)
select snmp as the Mastering Type
select the port that you are monitoring in the Interface list
check the Speed box and then enter the link speed in bps (speeds over 1 Gbps do not show the correct bitrates on graphs - probably a variable length issue?)
select a Type to indicate what the connection is (there are some pre-made link types, and you can also create your own)
On the History tab of the dialog, check "Graph Bit Rate"
As to which port you need to select - Dude can't really help you there. You'll need to either go to the site and look, or log into the switch and figure it out yourself (does 3Com have LLDP neighbor information or anything like CDP, etc???) Maybe examine the forwarding tables and find a MAC that you know is on that uplink?
Once you have the link modelled on your map it will go RED when the data throughput gets high (not sure what the threshold for that is and I don't think it can be changed. Once it has been running for a while double-click the link and select the History tab to see historical bandwidth utilization on that link.