OBMM was made before the Steam version, so it fails to detect OBSE under Steam. OBMM doesn't really need OBSE in itself, but some modders have added an OBSE check in the OBMM installation script, and that prevents the mod from installing when OBSE is not found. This check is usually among the 5-10 first lines or so in the installation script, and can be removed from the script.
So right-click on the omod and select "View script", then in the text editor that opens, look for something like this:
IfNot ScriptExtenderNewerThan
...
FatalError
EndIf
And remove it all. Then close the editor window, and answer yes to the popup that ask if you want to save. Now install without the OBSE check.