I assume this is when you access the store with an internet browser, not App Manager? The normal cause is ovi not detecting the correct user agent (browser) string.
To reslove this: if you have "Hide User Agent" installed, remove it. If you don't, look up what to insert as the UA string for microB - "Firefox/3.5 Maemo Browser 1.7.4.8 RX-51 N900". This has to be the UA string if you want to download dirctly from Ovi to your N900 with microB.

Another possibility is you need to upgrade your OS - what is the output when you press +#0000# in the phone key-pad?