To connect OBM to OBS through Microsoft ISA Server, please do the following to enable [Web Proxy] service on the ISA Server:
Open [Start] -> [Programs] -> [Microsoft ISA Server] -> [ISA Server Management]
On the left panel, select [Microsoft Internet Security and Acceleration Server] -> [SERVER] -> [Configuration] -> [Networks]
On the top menu, select [Action] -> [New] -> [Network]. Enter a “Network Name” (e.g. Internal) and define the IP address range for your internal network
(e.g. 192.168.1.0 - 192.168.1.255)
Double click the new network that should now be added to on the bottom panel
Select the [Web Proxy] tab
Check both [Enable Web Proxy clients] and [Enable HTTP] checkboxes and choose a [HTTP port] for the web proxy service
(Optional) Press the [Authentication] button, check the [Basic] checkbox and the [Require all users to authenticate] checkboxes if you want to enable and enforce proxy authentication
Press the [OK] button to make the changes and press the [Apply] button to apply the changes
Open [Online Backup Manager] from the system tray and press the [Options] button
In the [Proxy Seting] section, select the [Proxy] radio button
Enter the hostname/IP address of the ISA server in the [Proxy Address] text field and the TCP port for web proxy service on your ISA Server in the [Port] text field
Press the [OK] button to connect to the server

If you run into any problems connecting to the OBS from OBM using ISA Server, please do the following to check if your request is being blocked:
On the left panel of [ISA Server Management], select [Microsoft Internet Security and Acceleration Server] -> [SERVER] -> [Monitoring]
Then select the [Logging] tab that appears on the right panel3. Click the [Edit Filter] link and create a filter with the following values:
[Log Record Type] equals [Web Proxy Filter]
[Destination IP] equals [The IP address of Offsite Backup Server]
Press the [Start Query] button
Open [Online Backup Manager] again and try connecting to the OBS using the same procedure above.

Check if there are any errors generated in the log table.