How to calculate molar mass of (6)UO2+2D and (6)UO2OH+D?
These notations signify someone performed calculations in Visual MINTEQ accounting for dissolved organic matter (DOM) using a NICA-Donnan modeling method.
From the manual nicadonnan.pdf bundled with the installer:

In Visual MINTEQ, a “D” suffix is used to identify a diffuse-layer species such as a Donnan species.
[…]
The numbers 6, 7, 8 and 9 have no conceptual significance; they are only used by Visual MINTEQ to distinguish different humic components, and they reflect the order in which the components appear on the NICA-Donnan menu (with 6 as the starting number).

Considering the default numbering layout

and that FA1 and FA2 refer to fulvic acids containing carboxylic (FA1) and phenolic groups (FA2), OP's species can be interpreted as such:
(6)UO2+2D(aq): Weakly (electrostatically) bound uranyl $\ce{UO2^2+}$ to fulvic acid in the aqueous phase.
(6)UO2OH+D(aq): Weakly (electrostatically) bound uranyl monohydroxide $\ce{UO2(OH)+}$ to fulvic acid in the aqueous phase.
FA1-UO2(6)(aq): Organically complexed uranyl $\ce{UO2^2+}$ to carboxylic fulvate in the aqueous phase.
Several examples from the manual:

FA1-Zn(aq), FA2-Zn(aq): Organically complexed Zn to dissolved fulvic acid. Sites 1 and 2  refer to carboxylic and phenolic functional groups, respectively.
FA1-Zn(s), FA2-Zn(s): Organically complexed Zn to fulvic acid in the solid phase.
HA1-Zn(s), HA2-Zn(s): Organically complexed Zn to humic acid in the solid phase.
(8)Zn+2D: Weakly (electrostatically) bound Zn to dissolved fulvic acid.
Zn+2D(s)(6): Weakly (electrostatically) bound Zn to fulvic acid in the solid phase.
Zn+2D(s)(7): Weakly (electrostatically) bound Zn to humic acid in the solid phase.